You get energy automatically over time, from completing daily missions, as daily login rewards, or purchase stamina in the Recharge Shop. Tap on your stamina in the main menu to see a countdown timer on when more stamina will be earned for free.
When you level up, your stamina is recharged for free.
Maximum Career Stamina begins at 10, and increases by 1 stamina every level, up to a max of 60 Career stamina at the level cap of 50.