15 and 16, i remember my first girlfriend when i was about your age.
Try dry sex, fully clothed so no penetration, when youre in an 'intimate' monent kissing, touching eachother and cuddling, hold your body close to hers and try to mimic the movements youd expect with sex.
It might arouse her having you rubbing against her and can be physically stimulating even for both of you just having someone else pressed against your sexual organs.
Dont pressure her (i think theres laws agains having sex with 15 year olds) but perhaps the mood might strike and in the moment she might try touching you to feel you, or let you touch her - take it slow.
I started dating when i was about 16 and didnt sleep with a girl until i was about 19, but that doesnt mean you cant still both enjoy eachother without having sex.
Good luck