Op, if you want play destiny 2 for free, then the first thing your going to do is work on improving your gear score.
There are 3 main activity groups you can use to do this.
Vanguard = Pve
Crucible = Pvp
Gambit = Pvpve
I would stick to Vanguard and Crucible starting out as a new player as Gambit can be extremely frustrating.
Basicly you want to go to the tower and talk to the vanguard and crucible nps' and grab their bounties. You may also want to grab some bounties from the gunsmith too. The bounties help level up the npc, and they give you some decent gear for each reward tier.
Then to start off, run strikes and try to do all the vanguard bounties. You can usually take care of them in about 3 strikes. Makes sure you pay attention to the strike modifiers, as setting you class to match the elements in the modifiers can also give you a reward after 3 strikes. You not really going to understand what going on story wise, but just shoot the bad guys and kill thr boss at the end.
Now for Crucible bounties, I would stick to the 6v6 modes. Your going to suck, so your sticking to team modes will let you finish your bounties without too much stress. Your are going to die, and sometimes it's not going to seems fair, but as you learn the game and the maps, you will start the get better. If you start liking pvp and want to get better, you can train in the rumble mode (everyman for himself)
Basicly you are rasing your gear score so you can do harder activities like exotic quest, or nightmare strikes. Or if you really like the game, raids and dungeons.
You do have to pay for story campaigns, but you don't actually need them to get to the end game (but they do grant you access to new subclasses) and make the grind a little easier since the story tends to give better gear as the campaign progresses.
Also, as a free player trying the game out, you don't really need to worry about the Battle pass. You can earn the seasonal exotic on the free tier.
Destiny is a nice game playing solo, but it's way better if you have a friend
To play with. Good luck op.