Jazakallah khairan for your question. It is a very common question young men and women ask as this is a time when they are faced with increased hormonal activities in the body. To shed some scholarly light on this topic, here is an answer from Dr. Muzammil Siddiqi, former president of the Islamic Society of North America.
Muslims should have good relations with all people, males as well as females, at school, at work, in you neighborhood etc. You should be kind and courteous to everyone. However, it is not allowed in Islam to take a non-mahramperson or persons of the opposite gender as a very close friend. Such friendship often leads toharam. In the Qur’an, Allah mentioned that good men and women are those who marry, do not have fornicating relationships and do not have “paramours” orAkhdansee An-Nisaa’: 25, Al-Ma’idah: 5).
Akhdanare “sweethearts” or for a man a “mistress” and for a woman a “lover”. The Prophet, peace and blessings be upom him, is reported to have stated that“whenever two strangers of the opposite gender are alone with each other, Satan becomes the third one between them.”(At-Tirmidhi)”
So it is not allowed for a Muslim boy to have a girlfriend or for a Muslim girl to have a boyfriend. Howsoever pure your intentions may be, the danger is that it will lead you to sin. Or at least you will be alone with each other and spend more time together.
Thus, you should be friendly with your classmates, boys and girls both; but do not take a girl as your intimate friend.
If your friend, not girlfriend, is interested in Islam, by all means help her to become Muslim. Give her the Islamic books and ask her to attend Islamic meetings and lectures. Let her accept Islam by her own will. Do not force her or put any pressure on her to become Muslim. May Allah bless you and keep you on the right path.