Here were go, 2 sims away from live game action! So what do you need to do for the preseason? You need to at minimum do these things:
Go to depth chart and autosort all. Then you can adjust as needed. As I mentioned before, you might want to play a spare RG at LG or something. Or put a backup CB at backup SS. Auto sort doesn't play people out of position, but you can do it manually. But at minimum, autosort so that you have a lineup.
Go to gameplanning and hit "load recommended" for everything. The computer will input a gameplan for you. In the future, you can tweak the game plan yourself. Or you can hit load recommended every week. What I suggest is hitting load recommended until you get a win you really like then keep that gameplan and adjust from there a little at a time. If you read the box scores or watch the video you will start noticing things like "hey we stink at running" then you can crank up your pass percentage. Or notice that a certain play never works "hey that FB run never gets any yards." Well remove that play from your playbook. That's kind of how I do it. Read the gameplanning thread for more info.
One more task: Under gameplanning, hit "misc." Set these miscellaneous rules to your liking, but note the one near the bottom for preseason. Set when you will pull your starters in preseason. Usually I set it to pull starters after two quarters, but I am doing 3 quarters to start to get my guys some extra reps. The reason to pull early is injury prevention and getting your backups some reps. Reps matter - the more a player runs a play, the better he gets at it. You can see this reflected in "play familiarity" under your offensive and defensive plays.