Okay Julie. FIRST you're gonna hafta decide what you think you're gonna do here and that will control the visa you can get.
You can't work OR volunteer (legally) on an O-A visa or an O-X visa (both that you get from the consulate in your country before you wash up here).
An ED visa is a possibility but you should come here and tour some schools BEFORE you pay out your hard earned baht for a year's tuition and the paperwork to get an ED visa. you also cannot legally volunteer or work on an ED visa either.
I'd agree with
@Maxim **********
, get a 6 month METV from the thai consulate in your country before you wing your way here. That visa will give you 9 months of stay in country (in 60 day increments) and it should be more than enough time for you to decide the visa that "fits" your situation best. Plus you won't be shoe-horned into a visa that doesn't meet your needs either.