Easy Free Coins
You can earn 10 free coins by connecting your game to Facebook and/or Twitter. Alternately you can earn 30 free coins each time you watch an ad video. Ad videos will refresh every two hours or so.
Hard Free Coins
Completing the three game objectives you see on the roster screen before the game begins will earn you free coins. These tasks can be anything from scoring in the first half to limiting the opponent to a certain number of goals. Completing season and career objectives will also earn you free coins. Another alternative is to play scrimmage and tournament games, although these will not count towards your league ranking they are still a way of earning free coins.
Buying Coins
It is not worth spending real money on purchasing coins, you should be able to get a fair amount just by winning games and watching the ad videos.
Save Coins
Try to save up your coins for the transfer market so you are able to purchase skilled players that could end up being mainstays of your team.