You can buy boosters and cash using tokens or you can watch videos to earn them.

To access these videos go to Shop menu, then tap Currencies. You can also access this page if you click on a booster or the cash icon at the upper right corner of your screen.
Note that these videos are available all the time so if you do not see them please be patient.

Always make sure to have a stable connection before watching videos. If the videos get interrupted due to poor connection you might miss the reward.