At its most basic a skill is nothing more than a variable you increment when it improves.
The skill tests would then use the higher (or lower, depending on system you devise) skill variable value and the test would be easier, and each success would further increment variable.
Your first step is to outline what kind of skill system and then go from there figuring out how to track/store them, as there are many ways.
I was going to add a cool capx I made with an incremental skill but Construct2 is not allowing them for some reason.
You can grab it from my Google Drive though: https://drive.google.com/open?id=0Bx-ufu2WID53Y1I0VzM2RVZqTWs (click the download arrow in upper right corner of screen)