GTA Online is part of the GTA V and there are lots of things to do there.
first you need to make your own character. You can make it look like you wish. You can make your characters hair, eyes, and you can put on the clothes that you want to see on him or her. You can change your appearence later at the clothes shop as you wish.
After you did the tutorial you will be able to buy houses, garages, cars or rob stores and do missions. You can do exciting missions with your friends, crew members or with random people.
If you want to join a crew you need to open the social club form the menu and choose one or create one and invite your friends. Crew members can do missions and heists together. If you're a crew member you can invite other crew members to races, deathmatches and game sessions.
You can get money if you do missions. Look for missions with double cash to earn double money. The mission points are always shown on the map. If you buy a house you can go heists from the heist room too. Lester will call you if there will be a job to do. You will always need a car to get easily to the mission points.
In GTA Online you can spend money for cars, houses, garages, guns and facilities. There are some free things too for the starter players. You can buy the cars and the houses from your phone from internet websites. Guns are available from the Ammunations that are on the map. For starters the recommended guns are the micro SMG and the SMG. If you think you got too much cash you can deposit it at the ATMs there are placed on the map. If you buy things from the internet it you will need to pay from your money that are deposited at the ATMs
Other ways to get money:
You can get money if you rob stores that are on the map. There are more than twenty stores on the map but if you go back to one that you robbed the cashier will recognize you, so you can rob a store only once. You can also buy snacks from the stores to heal up when you got hit. Another way to get money if you steal cars and sell them at the Los Santos Customs. You can only sell one in an hour but there are for places where you can do it. You can also customise your car there.
Other things to do:
In GTA Online you can have a rest by playing golf or tennis or go to the cinema. If you would like to party you can go to a striptease bar or go to the Diamond Casino and try your luck by playing roulette, poker or play with slot machines or spin the lucky wheel to have a chance to win huge rewards.
There are daily objectives to do every day and if you do all the three of them you will get money. If you do the objectives every day you will get bonus money and for players who play every day it is very profitable.