Im a new student at school in the ninth grade and there is a girl i like what do i do?

Ok so im a new student in my school and there are about 250 students in my grade so the first time i have even spoken to here was on our recent field trip. we stopped to eat lunch and a couple of students went to subway to eat. While i was in the line a friend of mine from homeroom came up to me and asked if i would let her and her friend ( the girl i like ) order some cookies with my order... i realize now a good move would have been to buy their meals myself...but its too late i want to talk to her and get to know her but i never have time and her friends are always around her ..... what do i do? Also she doesn't know my name i kinda messed up with the intro. in the restaurant (she was super hot got kinda distracted) how should i go about introducing my self?