Go? The game is as simple as it gets. There is no need for happiness to level up your pokemon. To level up your pokemon you need stardust and that pokemon pre-evolved candies. For example to level up a raichu you'll need stardusts and pickachu candies. A Pokémon level can only go as high as your current level. Once you have enough stardust and candies tap the power up button. Notice that white arc above the Pokémon. It should fill the arc when you power up.

If you mean gold, there are some ways to increase the happiness.

Catch with a friend ball.

Walk a lot with the Pokémon in your party.

Level it up.

Use it in gym battles.

Give it vitamins.

You lose happiness if it faints in battles, gets traded, not happy with its haircut, get fed energy and heal powder, energy root, revival herb.

Which one is Pokémon? Is he the red one, the yellow one?....