"Troops" are your cards, the only way to obtain these is opening chests. The only way to open chests is with any of the 5-6 various types of keys. You will find gold keys randomly in all game play modes, but personally I recommend pvp. In pvp not only will you earn random keys, but you will also get glory. You can use this glory to open even more chests. Also, if you are in a guild you will get lots of benefits daily.
Of course, you could also just buy them, but I highly discourage that because the prices to what it takes to earn it in game are horrible. Best to grind pvp and get lots of benefits all around.