What kind of tree is it? Some wood types are too soft/sappy or toxic for a tank... You'll probably have to do some research to make sure that wood is ok, but the bark should be stripped before you prep the wood any further. If you use a metal trash can, you can put the rest of that downed tree to good use as firewood to boil the water in the trash can for a few hours--it'll get rid of a lot more sap & lower the risk of bringing "nasties" into the tank. If not, I would strip the bark, then soak it in a trash can full of chlorinated tap water & table salt for at least a week (in the sun), changing the water every day at first, then every few days after that. When the water stops turning brown, check the wood to make sure all the sticky sap is off the surface, then set the branch out in direct sunlight for another few days to dry.
I have another wood-related experiment going right now actually... A buddy of mine owns a BBQ restaurant here in Houston & he's got a massive hunk of oak that he's smoking for me. It's been hanging in a corner of his smokehouse (away from the meat) for about a month now & he says the grain of the wood is turning really cool colors. The smoke should be killing off anything on the wood & the heat caused all the sap to drain out in the first week or so. I'm gonna build a "pondless" waterfall out of it (covered with moss & anubias) once its done.