If it is driftwood, boil it for a while(time I don't exactly know), and then let it sit in some water for a period of time until it doesn't float any more. If it is already heavy enough to sink, let it cool off and put it in the tank. I don't know if the wood could go in a goldfish tank. Just wait to see what other members have to say.
I have wood in my Goldie tank (quite a bit of it actually)......the only thing it will do for a start is change the water brown as the tannins release from the wood...and those tannins will change the PH of your water so you will need to do water changes to keep that in check...
The best thing to do if you can is put the wood into a bucket (or appropriate sized container) and let the tannins leach out in that....when the water in the bucket is clear...then put in your tank (me I couldnt wait...so i'm battling the tannin coloured water)