I agree with Zagz - driftwood purchased at an lfs will have already been cleaned and sterilized. Soaking it for a few days will help make it sink in your tank, and it will also help release the tannins, a natural substance in the wood, that can make the aquarium water the color of tea. There is no harm in having the tannins in your tank, and to certain fish, it would mimic their natural environment. But a lot of people would prefer to get the tannins out before the wood goes into the tank, and that's fine too. You may have to soak the driftwood for a week or two in a bucket (aquarium-only bucket that has never had soap in it) and change the water every day until all the tannins leach out. Boiling the wood will work too, and will remove the tannins faster.
The plastic and silk plants sold in the lfs are safe for fish. They won't color the water or hurt the fish. But if you have fish with long fins, such as a betta or guppy, some of the plastic plants should be avoided since they may tear the fins. Use silk plants with those types of fish. You can also rub a pair of pantyhose over a plastic plant to test for sharp spots. If the plastic plant makes the pantyhose run, it will probably tear long, delicate fins.