If you really want to be certain you'll hit your prostate on the first
try, you can get an angled T-shaped vibrator that will aim straight
for your P-spot. Otherwise, slide your vibrator a few inches into your
rectum. This should put the tip of it near your prostate, which is
located between the wall of the rectum and bladder. Angle the head of
the vibrator toward your belly button so it presses against the inner
wall of your rectum. If you move it in and out slightly, you may feel
it rubbing against the prostate. Switch it on if you haven't already,
and use it to stimulate your P-spot, keeping pressure against the wall
of the rectum. You can also move it down and then up, so you get it pressed up "under" the prostate. As the prostate gets stimulated, you may feel the urge to urinate. This is normal.
Whenever you insert anything into your anus, make sure it has a flared
base or something to stop it from going all the way in. Use plenty of
lube, don't force it, and stop if you feel pain.