I'm sure it is different in different places. I know in Texas, minimum wage of waitstaff/bar staff is $2.13/hour. Kitchen staff make considerably more money -- the Minimum wage for them in Texas is $7.50, but due to supply and demand, they probably make at least double that in the bigger metropolitan areas.
Perfect timing... this video was just posted by a comedian whose YouTube channel is a favorite of mine. He explains tipping in the US very clearly, in a way that is easy to grasp.
BTW, this guy does videos about a fictional restaurant. He plays almost all of the characters in his videos. Check out some of his other videos.
CD