Fireball
The fireball is the item that will deal the highest amount of damage in the game. The fireball can take down a boss-type enemy in a single hit depending on how long it passes through the enemy. The fireball is launched from your character and it flies to the edge of the screen and will stay there, moving with you as long as it is active. It will not stay active for long, so take advantage and use it when you really need it. One of the most important aspects of the fireball is that it will also move along the same trajectory that you are; if you jump, the fireball will jump as well. Keep this in mind when it is active as you are able to control it to attack forward enemies.
Shield
The shield powerup works exactly as you would want when it comes to defense. You have full protection from any damage coming toward you for a short amount of time. Use the shield when encountering boss-types or any obstacles that you have trouble with. If you mess up a maneuver around an obstacle the shield will save you, but only for the duration of the powerup. The shield is the most common powerup so use them as you feel necessary, but know that they are not unlimited.
Heal
The rarest of the powerups is heal. This powerup will heal your character and restore you back to full health immediately. Use this only when desperately needed because they do not come around very often. The heal powerup is best used when entering a difficult enemy encounter or right before you are on the verge of death, meaning your last and final hit. These healing powerups are often located in complicated skywalks that make them difficult to reach, but if you can succeed you are set up with quite a bit of insurance.