make it as a relaxing environment as possible. Maybe watch some sort of romantic movie? when the characters in the movie are making out smoothly say like "hey that looks fun" if your gf agrees then try to go in for it :)
btw im 13 too so dont take my advice too seriously only if you rly think it'll work
also you may have already done it bc this was posted a day ago but even so you can do this another time too
good luck :)