How Do I Volunteer?
Don't be casual about volunteering; realize that you're making a commitment that will take time and effort. If you haven't already looked at "Things that you should know before getting involved" on the RISE website, do so now: http://www.nas.edu/rise/chap1.htm
If you live in an area served by one of MSA's 30 local societies, contact them first. Several of them already have outreach programs, and others may have an "outreach chair" who will help you find a teacher to work with. You'll find a list of local society officers here, and a description of operating MICRO programs elsewhere in the MICRO web page.
If your area doesn't have a local society, begin with the national network of teacher-trainers ("Associates" and "Leaders") organized by the Lawrence Hall of Science's GEMS (Great Explorations in Math and Science) program. MSA collaborated with GEMS to produce "Microscopic Explorations" which is one of over 70 GEMS manuals. They will know where you're needed, and can help you get started. If you live near a GEMS regional "Site", contact it first; they're listed at http://www.lhs.berkeley.edu/GEMS/sitescenters.html. If a "Site" can't help you, contact Terry Cort email@example.com to locate an independent GEMS Associate near you.
If none of this structure is close enough, return to the RISE website and explore it in detail. Very good advice is also available at http://www.noao.edu/education/ncmlssg.html
The North Carolina Museum of Life and Science produced an excellent pamphlet Sharing Science with Children — a Survival Guide for Scientists and Engineers; it's now available as a PDF at http://www.noao.edu/education/ncmlssg.html ; read it before you contact a teacher independently.
Finally, the Project MICRO Coordinator, Caroline Schooley, may be able to help.