See how to center clay on the potter's wheel in this free ceramics video.
So the first thing we need to do is center the clay. I've got that almost three pound lump of clay here for the actual house part of this birdhouse. Throw it down hard onto the bat. Now you're going to be putting your body weight through your left arm. You want to lean on your wheel head if you can or push your arm into your body. Any way that you can get stability and use your weight instead of your strength to stabilize this hand. Now you want to keep the heel of your left hand up against the clay. You don't want that to move very much. Your right hand is going to do the majority of the work. You want to get your sponge wet enough to where it's not dripping on it's own but if you squeeze it it'll give you a significant amount of water. And then you want to push towards your left hand with your right hand and pull up. Again, your left hand is staying stable. It's mostly only moving up and down. Now you make somewhat of a fist with your right hand and push back down. Sometimes you need to do this a couple of times before the clay will behave and get on center for you. But once you've got it on center like that you're ready to start making your piece.