At the very start of the game, once you can free roam, first unlock all the radio towers and do not use any vehicles. Doing this will naturally get you everything you need to upgrade your items, earn more money, and obtain free weapons. Make sure to loot everyone you kill and sell their items. Additionally, do side missions before doing the primary missions (for example, armed escorts, fashion week, etc.). The better head start you can get on your character before doing the primary missions, the easier and more fun the game will be.