You need to be an expert before trying to get involved.
FALSE. Your goal is to gain expertise! Professors do not typically expect that students will know yet how to help them with their work, and they may employ any number of methods to train you (for example, having you do independent training online or having one of their senior students train you).  Do not talk yourself out of trying simply because you don't know how to do everything!

You need to be at least a junior or senior (or the opposite, it's too late if you're a junior/senior).
FALSE. Some professors prefer that students get involved earlier, so they can learn more skills and become more useful to the project(s) by the time they are seniors.  Some professors only take students who have taken a specific, upper-division class, and some are open to having help for just a single semester.  So, just ask the professor what level of student they are looking for.

Professors are always happy to have volunteers, right?
It seems like anyone would love to have someone volunteer to help, especially if they weren't asking for pay or course credit, but it almost always takes time to teach someone how to do something.  For example, filing research articles is pretty easy, but you'd need to get the paperwork to get a key to the room where the filing cabinets are, learn that professor's system for filing articles, and know where to go if you run out of file folders.  The training for tasks takes even longer when the tasks become more complex, and sometimes the time it takes to delegate (i.e., train someone else to do the task) is much longer than the time it takes the professor to do it him/herself.  Be respectful of the investment a professor makes in training you to do new things.

The best way to get involved is to email a professor.
There are multiple ways to try to get involved, and email is the most common, but probably not always the most effective.  Ask in your major or your college advising office about ways to get involved.  Go to office hours and ask if there are opportunities available.  Talk with your TA's.  

If I don't get chosen, it means I'm not worthy.
FALSE. There are many, many reasons why you may not be chosen for a research or creative activity opportunity.  Learn to be resilient and gently persistent.  Ask what skills you can gain to be more marketable next time around.  Don't take rejection personally (this is good life advice as well!).

Find more tips on getting involved on the finding a mentor page.