Rule #1 - Forget real life. HD is similar in many respects but it must be treated as its own entity. For example, FCP offense and Press defense are played as full-time sets in this game. Substitution patterns quite often are 5 out, 5 in. Time-outs are thrown in almost randomly in many cases and are not used at seemingly appropriate times. There is no way to assign defensive stoppers to a specific offensive player other than doubleteams. There are other differences but these will give you an idea of what to expect. Play the game by the "game" rules and not real life logic.
Rule #2 - As noted, read aejones post on how to win at DII and DIII and also read the Coach Billy G. interview. From the My Locker page, click on the Editorial drop-down box then click on Interviews.
Rule #3 - Athleticism and Speed dominate this game closely followed by Defense. Key core areas are of course very important but not at the expense of those skills.
Rule #4 - As noted, go to the pinned section at the top of this forum and find the post on Mentors. Sitemail anyone on that list with questions. Also, ask questions on this forum. You will get good info most of the time.
Rule #5 - Have fun, it's a game.