1) You can check out plants here to be sure they're safe for your snake:
Edible and Harmful Plants
2)Basically you want to avoid cedar, pine, fir--the aromatic softwoods. They are most likely (esp. cedar) to contain toxic phenols that could be harmful. The hardwoods are generally safe.
3) If you bake the wood you choose at 250 deg F for 3-4 hours, you dry it out completely and kill any organisms it harbors. Just be careful to check it frequently, say every 20-30 minutes, the last hour or so to ensure it doesn't start smoking. If you see sap bubbling out of it, bake it longer until it no longer bubbles.