Are you asking how to equip/change apparel? Click your avatar on main page, then go to Edit Apparel at the bottom..
Are you asking how to get the Set Bonus? Make sure all your apparel is the same "brand", "rarity", and meets minimum level requirement. (i.e. all SE/Common/L4 or higher). You can't mix SE Epic hat with SE Common other stuff (although there are some glitches where you can get away with that). Keep in mind that the bonus is not guarantied, you just get a % chance to win that bonus. The % goes up the higher level your apparel is.
If you bought an apparel pack, it might be in your inbox and you have to go there to claim it. If you win them in-game they should show up right at the end of the game and you just click them to open.
Hope one of those answers your question, if not reply back with more specifics on what you are trying to do.