Ok, lets get down to business. I'm not here to make any uncomfortable or uncalled for comments...I'm here to help :))
I'm the same age as your friend so therefore I think I can help you out here. First off, as far as conversation goes, try to find something you guys have in common and talk about that and see where that goes. Also try learning some more about her but DO NOT pressure her into saying anything she doesn't want to. (not that you would just a warning though, 15-16 yr old girls...especially shy ones are sensitive sometimes about what they tell people.)
Also, try to make her laugh, not rude jokes though, girls like guys who make them laugh and fell comfortable. Be her friend as you already have been being. Maybe compliment her outfit or eyes or hair to let her know non-chalantly that you think she is attractive. (*whispering this* girls like that...:)...) Good job so far btw. :))
As far as physical stuff. Don't push it. give her hugs and arm around her. and then when you are saying goodbye if you are up to it kiss her cheek sweetly and casually.
Asking her out....hmmmm...this is a toughy! First of all don't be all typical dummy and say 'hey u wanna date' or 'hey u wanna go out with me?' Try something more along the lines of this...
"I had a lot of fun with you tonight, and I really like you. I think you like me a lot too. I know I'm a little older than you...I hope this doesn't bother you. So...I'm nervous in saying this...but...do you want to be my girlfriend?"
Then wait for her answer. I hope this helped...and ignore their silly comments. :))
