Praise be to Allah.

Firstly: 

We congratulate you for this blessing that Allaah has bestowed upon you, for by Allaah it is the greatest of blessings and it is guidance by means of which we ask Allaah to admit you to Paradise and join you thereby with the Prophets, siddeeqs, martyrs and righteous. Congratulations to you, for you have turned to Allaah at the same age at which many of the best of the companions of the Prophet (peace and blessings of Allaah be upon him) turned to Him, such as Mus’ab ibn ‘Umayr, ‘Abd-Allaah ibn Mas’ood, Mu’aadh ibn Jabal and Sa’d ibn Mu’aadh. Congratulations, for the record of your deeds has become white and pure, uncontaminated with sin. Islam erases that which came before it, so now you are starting a new life, a happy life in sha Allaah. We ask Allaah to protect you, make your heart steadfast and guide your parents, siblings and loved ones. 

Secondly: 

We praise Allaah that you have started the path of guidance by going to the mosque and studying the Qur’aan, for this is a good sign. The Prophet (peace and blessings of Allaah be upon him) said: “When Allaah wills good for a person, He grants him understanding of Islam.” Narrated by al-Bukhaari, 71; Muslim, 1037. You should do your utmost to memorize Qur'aan and recite it, pray at night (qiyaam), learn its rulings and act upon it, because in Paradise there are different degrees and levels, and it will be said to the companion of the Qur’aan in Paradise: “Recite and rise (in degree), for your status will be commensurate with the last verse you recite.” 

Thirdly: 

The attitude that your mother is expressing towards you comes as no surprise. Your real battle is with the shaytaan, who is not pleased that you have become Muslim, and he does not wish you well. Undoubtedly he will make use of your relatives and whisper to your family, so that he can use them as a weapon against you and prevent them from joining you in the true faith. Do not grieve or despair; seek refuge with Allaah from the accursed shaytaan so that his plot against you may be foiled. Be kind and compassionate towards your mother, for if she tasted the sweetness of guidance, she would not stand in your way. Seek the help of Allaah in calling her to Islam, be sincere towards her and make a great deal of du’aa’ for her, asking for guidance and mercy for her. Perhaps your du’aa’ will coincide with a time when prayers are answered, and you will have the joy of seeing her become Muslim. 

Remember that you are not alone in this matter. There are thousands of people whom Allaah has guided to the truth and who have chosen Islam willingly, out of conviction. Many of them have met with opposition and resistance from their families, then Allaah willed that their hearts should be opened, and the entire family has become Muslim. That will weigh in the balance of the son who was first guided. We ask Allaah to make you one of them and to grant you the joy of seeing your whole family become Muslim. 

Remember that opposition from family members is a test for the Muslim, so that the sincerity of his Islam and the strength of his faith may be made manifest. Allaah says (interpretation of the meaning): 

“Alif‑Laam‑Meem.

[These letters are one of the miracles of the Qur’aan, and none but Allaah (Alone) knows their meanings.]

2. Do people think that they will be left alone because they say: “We believe,” and will not be tested.

3. And We indeed tested those who were before them. And Allaah will certainly make (it) known (the truth of) those who are true, and will certainly make (it) known (the falsehood of) those who are liars, (although Allaah knows all that before putting them to test)”

[al-‘Ankaboot 29:1-3]

Among these examples of righteous believers is the great Sahaabi Sa’d ibn Abi Waqqaas (may Allaah be pleased with him) who used to honour his mother. When he became Muslim, his mother refused to eat or drink until he recanted his faith, but he refused to do so and remained steadfast in Islam. His mother found no benefit in her “hunger strike” so she went back to eating and drinking. It is narrated that he (may Allaah be pleased with him) said: “O mother, you know by Allaah that if you had one hundred souls and each soul departed one by one, I would not give up this religion of mine for anything. If you wish, then eat, and if you wish, then do not eat.” So she ate. 

See: Tafseer Ibn Katheer, 3/429 

Fourthly: 

The Qur’aan also deals with this problem, because it happened a great deal, especially in the first generation that left kufr behind and embraced Islam, and experienced intense hostility from their family, tribe and the closest of people to them. Allaah says (interpretation of the meaning): 

“And We have enjoined on man (to be dutiful and good) to his parents. His mother bore him in weakness and hardship upon weakness and hardship, and his weaning is in two years - give thanks to Me and to your parents. Unto Me is the final destination.

15. But if they (both) strive with you to make you join in worship with Me others that of which you have no knowledge, then obey them not; but behave with them in the world kindly, and follow the path of him who turns to Me in repentance and in obedience. Then to Me will be your return, and I shall tell you what you used to do”

[Luqmaan 31:14-15]

There is no room for obeying parents in kufr, or for obeying them in sin, but this does not mean that one should not treat them with kindness and respect, and take care of them, and do one’s utmost to guide them. This is indicative of the greatness of Islam, for it is the religion of compassion and love. Hence it calls upon its followers to guide people and bring them into the blessing into which they have entered. 

Fifthly: 

The one who has chosen the path of guidance should not pay any attention to what other people say and he should not be surprised by it, because that is something to be expected. What else will the Christians say about you? Will they say, “You have been guided and found the truth and you show it preference over your family and relatives”? No, they will never say that! So do not pay any attention to what they say, whether they call you a traitor or anything else. Regard them with compassion and strive to guide them, and strive to learn so that you can become a daa’iyah who can save them from their misguidance and deviation. 

With regard to your mother’s saying that you are inferior because you were born Christian, this is not true. Rather we say that you are our brother; you have chosen guidance and have returned to the fitrah (natural state of man) in which you were born. You were born a Muslim and a believer in Tawheed. The Christian scholars know that the child is born in a state of fitrah, hence they hasten to baptize the infant, believing that if he is not baptized he will become a Muslim! This indicates that the basic nature of man is Islam, and that if the child were left as he is he would be a Muslim.  

In Islam, no one is inferior to anyone else, except to Muhammad (peace and blessings of Allaah be upon him), because the Prophet is the Messenger whom we are commanded to follow. Apart from that, Islam teaches its followers to be free from the domination of priests, rabbis and monks, and states that a person has no need for any intermediary between him and Allaah. 

Finally, you say in your question that your girlfriend has accepted that you are Muslim and that you are planning to get married in the future. Here it should be noted that because Islam is keen for its followers to be chaste and for their hearts to be pure, it forbids men to have girlfriends. Islam does not approve of any such relationship between a man and a woman unless it be within the framework of marriage. If your girlfriend is a Christian, then you have to call her to Islam and strive to guide her. This will bring a great deal of good for both of you, in sha Allaah. If she refuses then tell her that Islam forbids this relationship between you and that you can never put pleasing anyone, no matter who he or she is, before pleasing the Most Merciful. You should either get married (and it is permissible for a Muslim man to marry a Christian woman) or separate, preferring to seek the pleasure of Allaah, may He be glorified and exalted. 

The most important advice we can give you is to treat your mother and father, siblings and relatives, kindly and understand their feelings. Ask Allaah to guide them, and choose wise methods of calling them to Islam, such as kind words, gifts, visits, inviting them to visit the mosque, and giving them useful booklets and tapes. 

We ask Allaah to take care of you and to help you to do all that is good. 

And Allaah knows best.