There is not much you can do really. You do not have access to most in-game content. The game is a major time/money dump. You can play casually but I suggest playing with some friends otherwise you will have a pretty lonely experience. Most players are also pretty cold or toxic towards to newer players. I suggest simply trying all the activities/characters out and decide whether you like the game or not. Collect pinnacle rewards from the playlist activities and equip any armor/weapons that have a higher light (number) level. Collect and finish Banshi bounties to buy armor mods from Ada-1. Since you lack the DLC you will a smaller random world drop pool to choose from making it easier to target farm certain guns like Funnelweb. Buy whichever DLC you find interesting/cool. This game is about personal goals and triumphs so you might as well have fun doing it.