Before adding driftwood to your aquarium, it is best practice to pre-soak the wood in a bucket (or other container large enough for the wood) filled with warm, dechlorinated tap water for 24 to 48 hours.
This will help to remove the tannins from the driftwood, as well as make sure it is waterlogged so that it will sink in the aquarium. If after adding the wood to your aquarium, you find that it is turning the water yellow, you can use carbon in your filter to remove the tannins, or repeat the pre-soaking process to remove more tannin before placing it back into the aquarium.
Natural driftwood may also, on occasion, develop a milky or fuzzy white bacterial film after being placed in the aquarium. This film is harmless; however, it is best to remove the film in order to prevent it from becoming unsightly. This can be done by removing the wood, wiping it down with an aquarium safe sponge, soaking the wood in untreated tap water for 24-48 hours, and then returning it to the aquarium. 15" to 24"