FUT-Guide-1 Each match you play counts down the number of contracts on a player, and then they're unusable unless or until you use a contract consumable on them. As such, using your silver and gold players early on is a pathway to the doom of your wallet.
As such, your goal needs to not be to build the ultimate team in that you're building the best team in the universe, but that you're building the ultimate team for the task ahead of you.
Quite simply, you want to run out there with not the best squad, but the squad that fits the lowest classification for as long as possible. This means using bronze players while they're cheap and effective. In general, based on the difficulty level, you can play well below the computer's team rating. Novice difficulty is playing soccer against an elementary school team. Amateur is only really slightly better.
The AI only really starts picking up in competence once you get to Semi-Pro, and I recommend playing on that whenever possible at least. But whenever you have an easier match? Run out the scrubs and reserves! The higher the difficulty, the less gap you have depending on your skills.
Top-flight, 4 star or above teams, start to get to the point where they're so physically dominant that unless you're a FIFA expert, you're probably going to get manhandled by them if you run a weaker squad out, no matter what. They'll win more 50/50 balls, get more corners, breakaways, everything, to the point that it's ridiculous. It will take some feel to know how bad of a team you can roll out there, but you'll start to get a hang of it. But use your best team as a limited supply of bullets that you only bring out in the most important circumstances. Bronze players are cheap and replaceable, if you can win with them you can save a lot of coins. FIFA 15 Coins will provide more coins to make you more convenient.
Thankfully, you don't have to build your team every single time you take on a match for different quality opponents. You can have multiple squads, and I generally like to have a squad with just bronze players, silver and bronze players, and then a gold-quality team. You can use the team builder to make this process a lot easier, but I generally recommend going in and playing around with the roster additionally beyond that. You might want to switch around your bench and reserves so that you're not wasting contracts on players just sitting around. As well, I recommend playing around with different FIFA 15 formations.
Sometimes you might get a squad with a higher rating and/or chemistry if you build it to different formations. If you're a junkie of the sport, you'll know what each means, but if you're a more casual fan, I'd say to go with the formation that makes for the strongest team. As well, while it may come at a chemistry deficit, sometimes it's worth switching out weak players at a specific position with stronger players who, are, say, midfielders, but don't play CAM position. In general, I'd say that you ideally want to run the most physically capable squad out there.
Don't spend your FUT coins all willy-nilly, as the only way to get them is to play matches and sell items. It helps keep the economy of the game in order since you can't directly just effortlessly buy coins and wreck the transfer market. If you're smart and handle your money wisely, you can buy more valuable card packs instead of spending money on dinky lower-tier packs. And remember: every card pack (the exception being that limited-time RED pack) can be bought with coins.
So don't be afraid to grind coins with bad teams if you ever find yourself short. You get coins even from losses, so every match has a reward – but optimizing your rewards is key.