If I want to ask a professor for volunteer or paid research opportunities for the fall, when should I start to ask? How do I approach the professor through the email? and should I still ask for paid position even though I have no previous research experience? Thanks for the help

I'm wondering the same thing.

Chances are the profs in bio aren't going to have funding to support a student. I would start off by stating the REASONS behind your interest in his/her field of study. Then email saying you would be interested in working in their lab. They're likely to respond (actually, it's more likely they simply won't respond, but don't get too discouraged, as you might get a response for only 25% of the emails you send out). that they don't have the funding to support a student. Then say you would be willing to take a position as a volunteer. Hopefully they'll respond positively to this. Seeing as you have no previous research experience, don't push your luck with a paid position. First step is getting your foot in the door. If you do that, and they see you're useful, you can then trying to negotiate some form of monetary compensation.
If it's the one prof that you're just dying to work with, then approach them face-to-face. You're probably gonna get the same answers, but you won't be playing email-tag for weeks.
Good luck.

One note of advice if you're emailing profs. Many of them use the same trick to gauge level of interest. They won't respond to a first email because they know many students just find a list of names and shoot off emails to everyone. If they see a second email, then they may respond. That being said, don't send that second email a day after the first one, give them a few days because they're probably busy people.

Thanks for the help. Should I start emailing them now, or should I wait until school starts? I cannot meet them face to face because I am not currently on campus.

Talk to profs. Most of them post it on OSCAR but it might be hard to find so asking first is the first step.

identify areas that you are familiar with in science. find those profs that correspond. email now. its very easy to postpone and forget... everyone becomes busy when school starts.

if you have no luck this summer, try again in september. its always good to get experience.

Start asking now

Some prof put up jobs on McWork, so you can look there too.

April/May (more positions will be opening up at this time since thesis students will be graduating)

I've always emailed, but in person wouldn't hurt if you're in town for the summer. Read a couple of their most recent papers so you can actually talk about why you're interested in their research in the email. I wouldn't ask for a paid position right away, just ask if there are any opportunities for you in the lab first. An unpaid position can always turn into a paid one later (I volunteered Sept-April in a lab and then was paid that summer from May-Aug).