Which screwdriver do I need?

Most of the IFIXIT guides list the needed tools as well as within the given step what the screw is.

Here's the IFIXIT guide to pop the bottom cover: MacBook Pro 15" Retina Display Mid 2014 Lower Case Replacement clearly in this case you're not replacing only cleaning ;-}

As to the needed tools, this is what you'll need for a screwdriver: P5 Pentalobe Screwdriver but if you don't have any tools at all I would recommend getting a kit so you'll be prepared for other repairs. And besides, you should get at least a wrist strap so you don't damage anything internally from ESD (static electricity). I would recommend getting this kit: Classic Pro Tech Toolkit