First play the solo challenges, play all that you can before any of your current players need a contract re up.... So play 14-15 challenges. Now BANK all the coins!! Don't spend any!!
Make sure to play the Lombardi challenge, you get 1000 coins and a Lombardi card. Next choose the regular season challenges they award cards sometimes and coins that progressively get higher each game.

After the 14th to 15th game take your coin stock pile and select search auctions. From here select players and search. Look for auctions ending soon 1-3 minutes. Look for small bids and players ranked 85+

Try to spend 500-1000 on each player.... Also look for buyouts that are cheap (under 1000)

Also remember that you need free space on your team to "buy it now" cards.... I missed out on getting cards that were worth 5000 for only 1000 buy it now because I didn't have an open slot.... By the time I made room (54/55) the card was gone.

Now you want to select a different position each time.... So DON'T buy 2qb's for example. After you get the players go to line up and hit Y to select the best players.

Now see what your rating is... If you need more just use the players you have now to play a few more games to buy the rest that you need.