Short answer: you pay for it with real money
If you don't want to spend real money and have no achievements your options are limited:
- You can watch video ads where you sometimes will be rewarded with a couple of sim cash. Something like 10-20 each day if you watch enough videos
- Joining the Contest of Mayors (and doing well). 2000 for winning highest league.
- Completing some of the quests also has sim cash as a reward, Can't remember which though
