How do I choose the correct airfoil for my UAV project?
There are two methods of aircraft aerodynamic design. One method is through using known airfoils (for wing and tail surfaces) and deciding other geometric parameters iteratively while meeting certain criteria for performance, stability, control, aeroelasticity, manufacturibility, etc. 

The other method is deciding roughly on the size and then attempting to come up with an airfoil shape, that (again) meets certain criteria. This second sort of design process is named inverse design. Where you try to get a desired Pressure distribution through computational iterations.

There are open source and free software as well as commercial options for both methods. Search terms: Airfoil design. 

Any general RC aircraft design guideline and relevant spreadsheets will speed up the learning process. Search terms: RC aircraft design guide. 

For detail information about the theory and practice; please refer to aircraft aerodynamic design textbooks.

Hope this answer sheds some light onto this wonderful and exciting topic. Feel free to shoot specific questions.