Here is a template I just made up, feel free to use whatever parts you want of it:
Dear ______(PI of the lab),
My name is ___________ and I am a x year undergraduate student at ________ (school). I am currently looking for an opportunity to join a research lab. In particular, I am hoping to ___________________ (insert statement on the lines of: learn more about a certain area you're interested in, develop skills in research you personally find meaningful, etc). I found your lab through ______ (whatever website you got that list from) and read several of your papers on __________ (topic). I find the work your lab is doing very exciting and I would love to have an opportunity to participate if your lab is currently accepting incoming undergraduate students.
Thank you for your consideration,
Whatever salutation you use,
Name
If you have a good GPA and specific experience in certain areas, advances classes (any upper div bios, non intro science courses, etc) you can always list that stuff at the bottom of your email.