It is very difficult for new players to do that, I would suggest spamming the sidejobs and maybe buying vehicles and selling them for higher prices. That is what I did and I made about 8 Million Dollars. I would also fish, sometimes you get lucky and can get about 2k per bag.
Here's a few tips I got when I started which helped me... so to sum the following things said before there is the
Spamming side-jobs... choose the ones which get you the highest amount of money, which are the trucking and delivery. some people prefer sweeping aswell.
Fishing and lottery tickets, just don't get obsessed over the lotto.
Advertising yourself as a mechanic or work as a taxi driver.
Here's the one who will gain you the largest amount of money:
Buying a vehicle/flat/business/anything sell-able, for relatively cheap price, and sell it for more e.g buy a Walton for 5k and sell it for 10k, that way you gain profit... do this for some time and you'll get a fortune gained by all the profits you've made.
To master that you'll need to know the average prices of the things you buy that requires experience on the server so if you doubt it don't hesitate to ask on the /n chat or PM a helper.