rain barrel-wooden

how do I keep a wooden wine barrel I`m using for a rain barrel,from leaking?

Well if it was used for wine it should already be tight enough. If not then you could try hammering the metal rings down to tighten it up. Also remember that water makes wood expand and could cause leaks. You could line the inside of the barrel with wax to keep it from absorbing water. Or you could do like they do for wiskey barrels and char the inside with flame.