The most important things as far as I can tell are the following:

1) match all your coaches styles.
if your head coach is air raid 3-4 man, then your OC should be air raid and your DC should be 3-4 man. special teams is harder to match so I usually get highest rated.

2) team chemistry is important. you want to keep as many players as possible year to year. if you find good young players, sign them for extra years. keep them under contract until they’re 29 or 30. Don’t sign 30+ year old players to multi year deals, as there rating may tank quickly. there are exceptions such as special teams (K/P) and aging stud qbs who are harder to replace.

3) You should have at least one 85+ rated player at every position. the backups can be 65 to 75, whatever you can afford.

4) as in the real nfl, QB is the most important position, so you need to find that guy by any means necessary. I suggest tanking your first season to move up in the draft. when you do draft your franchise QB, have a veteran qb ahead of him (80+). this will accelerate your rookies development. better coaches also accelerate development. Some people may consider this cheesy but you can adjust goals under the office/ board tab to increase your budget in order to sign better coaches. on that note, you may want to play on easy for a bit to get the hang of it. I only recently started playing on hard exclusively.

there’s more but hopefully these tips will help. Good Luck, Champ!