Penalties
Penalties are no more difficult than they need be. Saving them is a question of first deciding where to place the keeper's hands, before then sweeping your finger across when the player kicks the ball to initiate a dive. Taking them is even simpler, with a screen swipe in the direction you wish to shoot the only input.
Tackles
Watch out for sliding tackles. While it might look to you to have been a fair challenge, as with all football games on iPhone, the game can often take a dimmer view, handing out cards aplenty. If you can, try to win possession merely by pressing the player with the ball.