Make sure it's well aged too. Green wood needs to age outside in the sun and wind for a year before you should put it in a tank.
Any wood should be soaked for awhile in a tub or bucket, to remove or kill any insects that might be living in it, and to waterlog it so it will stay down underwater. Lake driftwood is already waterlogged, but if it's dry wood it can take awhile, weeks, or months before it will stay down by itself.
Also remove any bark.. it will just rot and make a mess. Many people boil wood for a few hours, which also kills insects, helps waterlogging along and removes some of the tannins which turn water brown or yellow. Leaching of tannins may continue for a year or two, no matter how much you boil or soak it first, but the longer it soaks first, the less leaching you'll see. Hardwoods are best, and generally, it's thought that evergreens are not good to use, some of the resins in the sap may be toxic.