Haha :-)
On your team page you'll see under the coaching staff what coaches you are missing. Under the search menu, you can search for coaches. If it's in early free agency, you can make an offer on any coach in the league - even ones on other teams as long as the offer is a promotion from their current position. Once early free agency has passed, you can only make offers to unemployed coaches. Coach hiring is done in any stage that free agency is processed (i.e. all stages except during the draft).
You will need a head coach in order to set your game plan; other than that, missing coaches will impact your player improvements during training camp. If you still have missing coaches when you enter the first preseason game, the game will automatically fill your coaching staff for you.