My iPhone4 got wet. And now the home button doesn't work.

First, don't try to turn it on or charge it.

Thoroughly wipe it off and place it in a baggie with raw rice and set it in a cool, dry cabinet. Leave it there for 3 - 4 days. After that connect it to your charger, charge it for at least 30 minutes and while it is connected to the charger Reset it.

I would place the iPhone in a big bowl of white rice, rather than in a baggie. That's what my brother did with his iPod touch that got wet, and it fixed it. The iPod touch works perfectly fine now, and the bowl is better because while the rice soaks up any wetness or condensation, the openness of the bowl gives way for evaporation.

A good idea would also be to take out your SIM card, if at all possible. Put it back in when you're ready.

I would do what previous users have posted but be sure that the phone is off also.

The whole idea is for the rice to absorb the moisture, which works far better in a sealed bag. Or you can use new silica gel packets in a sealed bag. If you want evaporation you don't need rice or gel packets, just set it in a cabinet and let it sit a couple of days longer.

Near an Apple Store? Make a Genius Bar appointment, have them look at it, and they will either give you a one-time forgiveness mercy pass or you will need to pay $199 USD and do an out-of-warranty replacement.