They generally sell two kinds of paint at the store, interior and exterior. The exterior paint is for painting things outside. It should be water resistant once it has dried and last longer under the harsh elements of the outdoors than an interior paint would. Most latex paint cleans up with water but there are some brands that are oil-based and you'll need a paint thinner if you intent to keep your brushes.
While you are buying paint, make sure you get a mid to high cost can of paint, never settle for the cheap stuff. It will take many extra coats of paint to cover what a more expensive "one-coat" can will take, which means you'll run out sooner and have to buy more, which will be more expensive and time consuming in the end. Another tip would be to buy more paint than you think you will need especially if you are getting it tinted a special color. That way, you have an extra gallon for touchups later.