Typically, you work through a company or a group. They do offer single volunteers to help and usually are glad to take any help as long as you are capable of helping.
Once you find your local branch, contact them to find out when the build days are or if they need help at their stores. They have stores for people who want to help but are not worthy of wielding a hammer.
You will need to fill out an information sheet for items like, insurance, contact info, etc. You may need to bring your own tools. I have been at some where they are provided and some where they are not provided. The local website should have information about it, or even better, just ask the contact person.
Overall its really easy to do and really is a lot of fun. I did it in school and tried to continue on afterwards, but I have recently moved and havent had a chance to go back.
Have fun.
edit: Oh yeah, you can be hired on through them, but it is an extremely small stipend and fairly tough to do.