I get my tree branches from the side of the sidewalk when I'm walking back from class (I'm a college student). Dead branches fall down after windstorms and are quite pretty sometimes. You want to get wood that is thoroughly, completely dead. And don't use wood from every tree species. Some of the trees (walnut trees, some evergreens) have poisons in them. In general, if the tree is one of the species where no part of it is poisonous, then it's safe to use in the fish tank. Just google the tree before you use its branch. If it has no poison warnings, it should be fine.
I wash them with bleach and thoroughly rinse them. You can't really boil the longer branches because they're simply too large to fit in a stovetop pot.
The wood sinks after about a day in water. I prop it in the first day, but after that it will stay submerged on its own. Wood grows mold after about a half a year or a year or so, so you want to keep an eye on it and watch for dots on the surface. Remove it when you see stuff growing on it and get a new branch.
If your water starts to turn yellow after adding wood, put fresh activated carbon in the filter. I buy my activated carbon in bulk and then fill filter bags with it by hand and it costs about a sixth of the price. The fish don't mind the tannins (yellow stuff) that leeches out of the wood, but I like my water crystal clear, so I filter with activated carbon. Actually, some fish (tetras, bettas) are more likely to breed if there are tannins in the water, I should mention.
Good luck with your wood. It's very pretty, I think.
Pictures of my aquarium:
http://img.photobucket.com/albums/v415/ ... /058-1.jpg