If you bought it from an aquarium or pet store it makes things easier, but just check there isn't a warning such as "not suitable for aquarium use" on any tags it came with. I'd give scrub it (in just tap water), boil it if it's small enough, then place it in a bucket for a couple of days to see if it changes the water colour.
If it does you basically have 2 options: keep soaking in a bucket and changing the water every few days, until it no longer changes the water colour, then add it in. Or you can add it to your tank straight away like newbietofishes doing normal water changes in your tank and the water will eventually clear up. Some people like water's colour when the driftwood changes it (wood releases tannins, and this is what discolours the water. It's not harmful to the fish, a lot of fish actually like it as it's found in their natural habitat. It's just a matter of aesthetics really)
If it doesn't change the bucket water's colour you should be good to add straight into the aquarium with no hassle.
Your driftwood might not sink either. I've heard of people who have had wood that hasn't sunk for over a year! That's pretty rare though. If the wood floats I'd recommend tying it down to some heavy rocks, rather than letting it float at the top.