it's a 3.6 1/2 AA lithium, a very common mac battery that a local Radio Shack should have for under $10. (if you are in the boonies, write me private and I can mail one for under $10 if you are in US.)
The battery is very easy to get to-check this page: http://www.academ.com/info/macintosh/veryearly.shtml
the plastic cage can be a drag- but the cap off a bic ballpoint can be used to pry the legs of one end out, then lift and repeat other end.
You weren't very specific- what exactly is happening when you try to boot?
Grayscreen with a "?" question mark means it can't find a system folder, boot from a compatible system emergency disk (7.5.5 disk tools, or 7.1 with the right enabler?) and run Disk First Aid on the HD. Then install or reinstall a system. 7.5.5 works well, if you have extra ram. If it doesn't have extra ram 7.1 has less overhead.
Chimes of death, then "sad mac" face or some hexidecimal numbers....
write the numbers down...
remove extra ram, any extra stuff plugged in (external drives, PDS expansion boards, etc) and try again.
Then sell it for parts, or ?