Shin guards, socks, cleats, soccer clothes (shorts and comfortable shirt or jersey), and optionally, a mouth guard are the obvious ones.
Some teams will provide practice balls, but I recommend bringing a ball to your first practice just in case.
You should definitely bring lots of water with you.
You may also want to bring a small towel that you can use to wipe sweat, put ice in to cool down, wipe mud and other debris from your clothes, and even sit on during warm-ups if the grass is wet.
In addition, if you coach allows it, you might want to bring some sort of fruit (like grapes, oranges, or watermelon) in a zip lock bag. This will give you a good energy source.
Finally, you should always bring a good attitude!