How do I email a professor asking him/her for research experience?

Keep in mind that undergraduates have a hard time getting these positions, and the professors are paid to work with them. They are not getting paid to work with you, and even undergrads aren't useful - you're asking someone to give up a large amount of their time with basically no return. Focus your request on one professor, someone in your area (not someone famous you saw on TV), read their website, try reading a few of their papers, and make sure you sound very interested, willing to put in the time (up to 40 hours a week), and willing to do whatever they need (cleaning glassware, data entry, etc - many undergrads start out that way). List any relevant skills (math and science classes you've taken, programming languages, anything related to the field).