Good people, "Don't Be Scared". Mirepoix is a simple blend of carrots, celery, and onions. There are a variety of uses for this base ingredient in many recipes that provide comfort and salivation. Most often it is used sauteed in butter, but may also be used roasted, or raw. Wikipedia does a sensational job with MIREPOIX. Don't miss this link. Take a break and go there now.
I think one of the best ways to learn how to use and ingredient is to taste it. So get out the carrots, celery, and onions. Dice them up (be sure to watch Chef Jacob Burton's great video - only 1.5 minutes - DICING ONIONS). Start sauteing them in some butter.
Watch how they cook. Stop a bit into the saute and taste them. Then, smell them, note the colors as they change and taste them all through the process and the changes.
As you are doing this, with this set of ingredients and any, ask yourself: "What does this remind me of?" "What would this taste good with?" and "What would this taste good in?". This is how you build the base of your technique and style. You run your ideas past your inner palate and then you experiment, experiment, experiment.
Give it a whirl.