Ok, what a special ability gives you is a % boost to a stat or 2 in a specific situation. The more you put into a special ability, the bigger bonus you get (and the more often it actually works).
Now, if you have a low stat, you will basically get almost nothing from any special ability, so focus on your primary stats first.
For a special ability to work reasonably well (to the point you might be able to see it making a difference), you are looking at having at least 5+ in it (preferably 7+).
For a DE, Shed Block is a VERY good special ability (and for a speed rusher first step and change direction are useful to keep your speed around a blocker).
Have a look at the mechanics faq for an idea of how the special abilities work.