|Near the bottom of this link you'll find a basic model release. Once you have a signed copy, you're good to go.
It depends on what you want to do with the images you shoot as to whether you need a model release. Images used for editorial purposes or non-massed produced art do not require model releases. Portfolio pieces that are not broadcast (ie, included in mass emails) typically do not require model releases. However, images used commercially (either on mass-produced products like calendars or coffee mugs, or in advertising campaigns), do require model releases; the only exceptions or when the faces are not readily recognizable or when the image has more than just a few faces (the number of faces that make up a crowd varies from state to state).
So, if you're just out shooting, it doesn't really matter. If you shoot something that you think you could license for commercial usage, you should be ready to whip out the basic model release.
The only other situation you should be aware of is shooting private events, either for hire or as a guest. In addition to any model release requirements, you might also need permission from your host, as well as the property owner, to license the images for any usage (including editorial), as well as to use in your own portfolio.