The single player campaign pitches you against 50 Goblin villages, each more difficult than the previous one. This campaign is available to all players right from the start of the game.

In the village view, just tap the 'Attack' button to find the Single Player campaign on the right side of the screen:

Every successful single player battle rewards you with gold and elixir, but the loot is finite for each level. The rewards grow as you progress in the campaign.  You only need a 1-star victory in a level to progress to the next. In order to get all 150 stars and unlock the achievement "Get Those Goblins!", you need to score 3 st