Unlock all the coin boosts that you can through the catalogue immediately, starting with the biggest boosts first (click the right stick in to activate the catalogue once you're in the FUT mode menu screens).
If you don't have any credits to spend on them - it makes life much harder, but you can still do it...
A 100 chemistry gives every player a 10% attributes boost - not enough to make a silver squad great. So concentrate on gold players - even if it means a middling chemistry - but most importantly, focus on one nationality or one league as soon as you can.
Take a look at your best players, write down all their positions, and use a formation that allows you to play all of your best players from the start. Build on that.
If they're mostly from one nation or one league, look in the transfer market for the cheapest auctions (not "buy nows") for players who are "gold" quality, from that nation or league.
Once you've got an ENTIRE gold squad, of one nation or one league - including a full subs bench of the same - then just hammer them for weeks until you get 8,000 to 10,000 coins. At that point, you can go for the much-better players in the positions you need them most, one by one.
Never buy packs. Even special edition packs focused on a high-level tournament like the Champions League will feature some awful players from awful Champions League teams. Even if you get a great player, they're not guaranteed to match up with your squad nationality / league, and selling them on, you usually won't make your money back.
That's my advice, anyway...
EDIT: Another great tip I always forget - manager bonuses stack (up to 50%). Some managers have a percentage that gets added onto contracts - so if you offer a player a contract, and you have a manager at your club (they don't have to be selected to manage your squad - in-fact, I never have a manager set, because they provide such a small chemistry boost), a small amount is added to the contract.
So if you have 25 managers with a 2% contract bonus each, your players will get a 50% contract length bonus with each contract you apply. An 8-match contract card turns into a 12-match contract card, for example.
Every contract you apply without having that manager bonus is extra matches wasted...