When we went, we found the Rough Guide very useful.
Chang Mai is a great place to make for - overnight trainfrom Bangkok. We did a three day trek from there in the jungle with bamboo rafting and elephant trekking part of it. Find Eagle House in Chang Mai and use them - they're more ethical / ecologically minded.
A day's bus ride north is Pau (an old hippy hangout that hasn't moved on): a great place to chill.
If you're happy to, hire a car in Chang Mai for a one week journey driving round the Mae Hong Son loop. (Car hire is ridiculously cheap.) That has stunning views to Myanmar (Burma).
Innnoculations: depends on what you're doing. We had tetonus, hep A and hep B. You'll also need anti-malarials. See your travel nurse at your surgery.
It's a wonderful place. You can eat and stay really cheaply if you're so minded.
Have great time.