I remember when i was at this stage so i understand how ur feeling Everyone has something unique about them maybe she has kool hair style, tattoo a book, a funky walk. Look at her think about something that defines her like any of the above and just mention it. "Wow that is some massive book u must b smart whats 2 x 0...haha this is a trick one u won't get it." then just carry on, "so what brings u in this area, i know where a kool libary is if u want lol".
So just do that and u will b fine, things to keep convos going
opinion: do i look better with the glasses or without the glasses
situational: wow why r we the only people in the train, did someone say its going to crash and i didn't hear it, its ok i can hide behind u
What shes wearing or holding: like book example
What shes doing: dam that exercise must b hard, could u always do the splits
or wow u got a really relaxed look about u its kinda kool...wait u better not b high lol
How she talks: awwww u r so quiet its cute, how old r u, stop me when i get close? 12, 13,14, 15
Future guessing: ur eyes look greedy let me guess ur an accountant, dam u look very discplined let me guess teacher
average convo (don't really like this but for starting off simple) hey how r u, what u doing work or college?
Dude going to b honest first few times not going to lie ur going to screw up a lot, but guess what thats the biggest point ur going to improve. Experience dude thats what u need and the only way u do that is trying out what u got then changing it every approach u do to something better. GOOD LUCK