i'd say someone rated 800 or below would be a beginner...
edit: my grammar looks strange.... 
ratings question from beginner
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Hi, welcome to chess.com!
The default rating that all people start with is 1200, until they complete a game.
You can go to the members list which shows a list of all members, but more importantly for your question also shows a graph of how many people have each rating. The average rating on the site right now is 1383.
The best way to find out what rating opponent you play would be to play some games and see how you do. I know that might sound weird, but everything is relative. 
It's no accident that the average rating isn't far above the default value of 1200. A good rule of thumb is to look for people who are within 200 points of that default, so I recommend using a range of 1000-1400. If you lose, then look lower, and if you win then look higher.
Edit: my reply took too long to type, someone beat me to it. The other poster's idea of 800=beginner may be accurate, but it's hard to say because that's a pretty vague term in chess.
A quick explanation of ratings: The rating system guesses what your score will be based on your rating and your opponents. If you do better than that, your rating goes up, if you do worse than that your rating goes down. A long explanation of ratings: http://blog.chess.com/ichabod801/a-plain-english-explanation-of-glicko-chess-ratings
I second Dave's suggestion. Play near your rating, if you do badly, play weaker people.
I am new to chess and would like to play beginners. I have been unable to find a quick explanation as to how ratings work. What is the lowest rating (beginners) so that I can request to play only those who are at my (verrrry low) level. Thanks.