Sorry to say this, but the answer can get quite lengthy, so I'll give you a condensed version.
The apparel only works in H2H coin games, and right now clashs. Each item has an explanation as to how it will help you. Ie. Hats usually deal mostly with driver distance, each has a level value attached to it. By winning coin games, you achieve points toward different items, randomly selected. The higher the level, the stronger the power that each item has. At the top of your cc home page, are 9 dots, each win fills in a dot, after 3 wins, you will get a silver apparel pack, which opens better items. 6 wins gets you a gold pack, somewhat better than the silver, and 9 wins get you a platinum pack. You can upgrade each item by using coins. Each item carries a rating, common, rare, and epic. Obviously, epic is the best. Purchasing most apparel comes with a count usage, 60 credits for 100 hits, etc. Purchasing apparel packs, usually does not go well for most players, there has been threads in here about the poor benefits that you get for your credits spents.