You have two options:
- Use real money, buy some packs to start you off
- Grind until you have enough coins to trade players
Personally I always choose the first option, it's stupid the amount of money that I've spent on packs, it's only £10 here and there, but I could probably have bought about 4 copies of the game by now. I'd recommend just spending a tenner to begin with, you'll probably get someone worth a few thousand coins at least and then you can crack on with trading other players hopefully for profit.
The only other way really is to begin with a bronze team, grind the offline trophies to get some coins, when your players run out of contracts you can discard them for a fixed amount of coins, then try buying some online that are equal to, or less than the amount that you can discard for. Not sure if you can actually do this with bronze cards as the minimum you'll get a player for is 150 coins and I think you only get about 60 coins for discarding a bronze, though I may be wrong about that.
Essentially, it's all going to revolve around luck to begin with, you'll have to be looking at the right auctions at the right time, then you'll need to sell at the right price, at the right time. It's quite tough to begin with and depends on how good you are at trading and how much patience you have. I'm absolutely terrible at the trading aspect, I can never seem to make a profit because I don't have much patience and I've rather use the easy mode of just putting a bit of money into packs here and there.