Im 17 and i was wondering how do u get your penis ready for sex?

Basically, you don't have to "do" anything.  It will respond and nature will do what it has to do.

If you are not circumcised, it may be a bit uncomfortable if you can't slide your foreskin back easily when erect.  Stretch it if you have too.

Use protection.  Both your bodies will likely produce lube, precum for you and she'll get "wet."  Don't rush things, take your time.

Hope it helps.

I prefer a clean penis, and by clean i mean shaven, and not smelly. So trim up wash your penis, balls and butt hole. (yes I said butt hole, you never know) and have fun! Although no matter how cool it might seem to have sex at 17, you should really wait till you get married and save that special gift for you spouse, it will feel 100 times better then it will if you do it so early, just think about it.