#1 thing: don't do this as a surprise. Anal is never a good surprise.
What to use: your finger, lube, a glove if you want.
What to do: Warn him. Let him go to the bathroom and make sure he's ready. Then you stimulate his penis somehow. Get him very excited (it makes the prostate firm up and become easier to find). Find a comfortable position where you can get at his anus and penis and he can relax. On his back, knees bent, feet near his butt often works well. Continue to stimulate him, but don't bring him to orgasm. Take your lubed finger and run it around his anus to allow him to get used to the sensations. After a while his anus will start to relax again, then you can gently insert your finger pretty much all the way. Feel towards his belly button for a bump - that's his prostate. Try different stimulation to see what he likes - forward/backward, side-to-side, pressure, etc.