If you roll exactly your skill, it's a critical, regardless of what your still is. That's why skills above 20 are so awesome = you add the surplus to the roll, and if you get 20 or better, it's a crit.
If you roll a 20 (and your skill is less than 20) that's a fumble.
In general, low numbers are better. In a competing skill check, lower than your skill is better, but you need higher than your opponent.
That clear it up?