If you got it dry you'll need to leave it that way until you're ready to put beer in it. Give it a smell, if it smells sour at all you'll want to know as you'll be working with a funk barrel.
Several hours before you're planning on putting beer in it, fill it up with water. Be prepared for it to leak a lot, so set it in the sink or in a garage or something. Your goal here is to get it the wood to swell and fill in all the cracks in between the staves. Keep adding water as it leaks out so it stays full. Once the barrel stops leaking you can drain it and put beer in it.
Make sure your beer is on the higher alcohol side. Like 8-9% and up. The reason for this is that a lower alcohol beer won't ward off infection as well as a higher alcohol one. A porous surface such as wood leaves a great deal of room for infection.
Last but not least always plan to keep beer in the barrel. If you empty it and let it dry on its own, the funk is going to move in. It'll be good funk, so if you're into sours and guezes and such it's a good route to take. But once a barrel goes sour there's no turning back.
Should I sanitize it? just fill it with water? leave it dry until I"m almost ready to transfer the beer over?
- Don't try to sanitize it. Its a porous surface so it's essentially impossible to sanitize. That and any liquid in the barrel is being imparted with good barrel flavors and the barrel is being imparted with flavors from the liquid. If you put sanitizer in there you're making the barrel taste more like sanitizer and the sanitizer taste more like oak. Which isn't really good for either.
Barrels are high maintenance but produce awesome beers that you can never quite get with the chips. I hope all goes well and good luck with your barrel!