The above guides are great, and I just want to add/reword a couple other ways you can build a good team.
Try and save coins early in the NFL season, wait for lots of new players to be added to the collection of MUT cards, and buy specific players from the auction block. I was able to build a 94 Ovr team within a few weeks casually using this strategy, without spending any money irl, albeit starting after the release date of Madden 17 was announced. When scouring the auction block, try and look for players who have good stats but aren't that well known or famous. These players tend to be just as good as the famous players, but can be bought for a lot cheaper. Players like DeVante Parker and Sean Smith are good examples of this. An easy way to make money is to put bronze, silver, or gold players in their respective sets in the Warehouse Set and get Badge Packs off of them. You can then upgrade the badges in the Vault Set. Completing Solo Challenges and Draft Champions are the easiest way to get free packs with lots of players. Once you attain badges, you can sell them off the auction block yourself. Gold Badges and Elite Badges sell the easiest, with Gold Badges being the better bargain of the two. NFL Journey II and III are the Solo Challenges that offer the greatest amount of coins and packs from my experience, so try and get through those early, get used to your team, and build up income before heading to MUT Seasons. I cannot stress using the auction enough, as you can add specific players and styles to your team, instead of using the unreliable (and expensive) randomness of packs and completing sets. I also wrote a guide for the The Champion achievement, in case you want general details on how to face human opponents online. As irritating as it is with how little effort EA puts into this game, MUT is actually a really enjoyable and competitive mode. With a good team, this achievement is easily attainable and quite enjoyable along the way, especially when you manage to get people to quit in the 1st or 2nd quarter. Good Luck and Have Fun!