Welcome to Los Santos. There's many options for how to start your journey, but the most obvious way is to get a job. Press
ESC to open the map preview, then click on it to open the full map. In the top right is the map legend, and you can select the brown briefcase icon to find available (legal) jobs.
When you first approach a job location, you will see a rotating question mark. The marker will prompt you to interact with it in order to accept the job. Accepting a job does not require you to resign from any existing fulltime positions (i.e. if you're a police officer you may still complete job missions).
Some jobs require a specific vehicle. If this is the case, you will be asked if you would like to rent one or provide your own. Rented vehicles require a non-refundable deposit, but you will be allowed to keep the vehicle for the rest of the day (until it's impounded or the server restarts).
Each mission has specific objectives that will be communicated via prompts after the mission begins. Check the individual job pages for more detail. If you stray too far from the job vehicle, you will receive a warning to return or risk abandoning the mission. You will forfeit any outstanding paychecks when abandoning a mission.
Each job pays in its own unique way. Some pay only cash, and others will direct deposit into your bank accout (if you have one). Some jobs pay incrementally as customers are served (i.e. the food truck) while others require completion of the entire task list before you are paid (i.e. trucking). Read the individual job pages for more details.