When you start a model you want to paint the it first, but not build it (unless it has paint already on it). You can buy paint at almost any hobby shop which the paint I would recommend is "TesTors" model paint. Then when you are done painting, which should take a day or 2 let it dry.
After you paint the model you want to start building it. When you build the model it is either a snap together model or a glue model and the B-24 Liberator is a glue model so I would recommend TesTors model glue.