For the well being of your fish i would suggest purchasing "drift wood" from your local fish store. Wood from your yard could be carrying unwanted lifeforms into your aquarium. Driftwood is simply dried wood usually attached to some sort of rock base to sit on the bottom of your aquarium. You can use it to grow live plants or just for decoration.
I just got a 3 pound chunk of Malaysian driftwood from Petco.com for $15 after s&h. Looks very nice. Easy to shop around online, but definitely boil that puppy once you get it so you don't leach too many acidic tannins into the water (unless you want brown, acidic water).
Last week I found a cool site about making your own driftwood - but I can't find it now d'oh! You basically have to let it sit in salt water for a couple months, and you have to use a hardwood - NO pine.