Ok so if you are serious about sexting both of you need to be willing and old enough (seriously if you're both like 13 you aren't old enough. ANYTHING "sexy" you send if you're under 18 isn't legal and can result in major issues for you or who you send it too.)

Reply to a comment like "sexy" with something like "Really, what's your favourite part?" Or "thanks, I'll bet you can't guess what I was thinking about when I took it." Be playful and innocent while making tons of reference to him and sex ... like "it's always SO Hard when I'm close to you." Basics are keep it light and playful until things get rolling and then take it wherever you like. If he doesn't respond well tease him playfully with a pout and ask "don't you want to play with me? I really want to play with you right now. But I guess I'll just have to play with myself, all alone."

My personal favourite to start sexting with my SO is when she sends something like, "baby all I have on right now is a pair of heels and a smile."