There are a few different gasoline tank sealing products. Jb weld being one of them. They work for small holes and cracks. I've had best luck with jb weld however none were a long term fix. Drain the tank you'll need to clean the area around the leak. Use sand paper or emery cloth. Do not use anything that could cause a spark. Mix and apply the product as directed. Good luck and be safe
If it's a fuel tank for gasoline, they have a gas tank repair kit at any auto parts store. They have them for metal and plastic tanks. They have a repair kit made by permatex for damage repairs and a in-tank sealant called redkote.