Depending on projector's capabilities...run an HDMI cable from MBP to the project. The MBP will likely adjust it screen to the max resolution of the projector. If the project does not have HDMI, it likely has old fashioned VGA port. For my rMBP I have a Thunderbolt to VGA adaptor into which I would plug in a VGA cable from the projector. My rMBP then lowers its resolution to match the project's resolution.
You can shop for a projector online at B&H, Amazon...etc. or visit stores such as MicroCenter, BestBuy, and office supply stories.