I reached level 30 and can't level up any further

When you have reached level 30 you can choose to prestige.

When you prestige you will level back to level 1, but your level emblem will change and you will be able to unlock new power ups, profile icons, etc.

To prestige, click on your level icon (30) in the top left corner, and a pop-up window will appear where you can click on prestige.