How to obtain coins
You can obtain coins through many ways. The easiest way is that every time a new even occurs, you will get coins just by clicking on them. Once you do that, the next batch of coins will be earned by playing games. You will earn a certain amount of coins based on your performance in the game and then some bonus coins if its your first win of that event. However there is a set limit on how many coins you can earn from each event. You can also get coins from the brawl box or a coin booster than will double the amount of coins you own for a limited time.
What to do with coins
Coins are the primary currency in Brawl Starts but can only be used to by crates. These crates will contain a variety of prizes including coin boosts, upgrade tokens, coins, and brawlers.