To play GTA Online, you must first complete the introduction of the single player campaign. It only takes 20 minutes.
You'll know the prologue is over when you see a cut scene of Michael, one of the main characters, sitting in a therapy session. Skip the cutscene, open the main menu, and you can join GTA Online and start creating your character.
GTA Online is a huge game that can seem very confusing at first. Embrace it and know that, as you continue to play and familiarize yourself with all of its activities and systems, it will start to make more sense.
Do tutorials. It takes about an hour, but the tutorial, in which Lamar takes you on a few missions, is an important introduction to various aspects of GTA Online.
The two biggest progression systems in GTA Online are Cash and RP (Reputation Points). Cash is essential for buying new guns, clothing, and assets, while RP increases your overall level. From the start, many things will be locked until you reach a certain level.
If you don't want other players to attack you, you can turn on passive mode in the interaction menu (press 'M'). It makes you invincible to other players but also prevents you from using weapons or attacking
Often deposit money at ATMs. If other players kill you, you will forfeit a portion of the money you have. So make it a habit to leave the ATM from time to time (use the interaction menu to find the nearest one) and deposit your cash.