Dress neatly. Dressing well is very important for first impression. Dressing well from head to toe is very important for winning over a person in their first impression.
Make eye to eye contact with that person. This shows confidence. Speak about all the good qualities you have. Speak about things you are good at. Speak about your interests.
Be very cautious about what you are speaking and what you are doing. Manners matters a lot for the first time. So be very well mannered and use only very dignified and nice words. Don’t try to be very casual.
If you have already seen this person and know something about that person, then speak in his/her language. If she/he likes to read novels speak about some authors and their books. The good things you liked about these books. The bad part of these novels. The climax etc…
If you know that person’s best friend, this is the best way to approach a person. Get to know everything possible about this person. Positives, negatives, life style, background etc… and take advantage of your knowledge about this person. By saying advantage, I only mean, speaking their language and doing things what they like and attracting and getting to know them in their own way.
Try not to boost about yourself. You may end up looking stupid in front of them if you are really not good at those things. Speak only about those things which you are very good at and if you do not know something, be matured enough to confess that, you do not know anything about that and take them to your comfort zone.
Never compromise on your morals and lessons you have learnt in life. These are the whole basis of your living. So try to make them understand this, instead of jumping to something which is against your own morals.
Be as formal as possible the first time. Nobody wants you to be very casual. By being very casual you may loose the first impression you would make on a person.