There are basically two ways you can control you character: either by tapping or dragging across your screen. Although it does not make much difference in the gameplay, the first option is generally better, because your finger blocks the screen only for a shorter amount of time.
On the other hand, if you plan on just shooting straight ahead of you and making smaller aim adjustments as you go, the second way is better.
Various weapons can shoot in different directions, so choose the ones that cover the most ground, or a combination that will both let you fire ahead and protect your back.
Keep an eye out for gold Bloons and try to hit them as soon as you can. They give you special powers that you can use to take out all the rest of them.
When you complete all of the upgrades for your weapon, you’ll get one final - epic upgrade, which makes your weapon extremely powerful. Use it to wipe out those pesty enemies.