A safe and easy way to earn money is to get a regular worker job. You can have up to 5 worker jobs (10 if you have VIP) at once. You will be able to complete a job once per 24 hours, and each time you complete a job you will be paid your wage by the company. You can look for jobs under Business » Available Jobs. On the Available Jobs page, click on the company name where you want to work. If jobs are available, there will be a box that lets you apply for the job.
Once you are employed by a company, you can complete your day's work by clicking on "Begin Work" button on the company page. There will be a notification on your Dashboard whenever you are able to work again.
Each time you complete a job you will lose 4 percentage points of energy. You can regain energy by consuming a Food Package, which are buyable from Retail Stores. More information in the article Energy & Retail Stores.
Note that VIP players can have up to 10 jobs per day, and by buying a boost drink from a retail store and consuming it, you will get to work 1 more job during that day.