You win by getting 50/75/100 points (depending on mode), every enemy you kill drops a point that you walk over to collect. The objective marker is the goal where you drop points off. The time limit is increased by 10 seconds for every point you drop off, decreased by 10 for every point the enemy mobs drop off. Play the 50 point count mode to start with; the 75 point count mode is more efficient but not by much & is much more difficult. Not many people bother with the hardest mode.
There's only two things you need to know:
If you want to be able to do Index yourself or pick up points in a pub game, I highly recommend bringing Rhino (Nezha works too). Carrying more than a couple points removes almost all your HP/shield, while Rhino's iron skin stays just as strong. He's the only one who can tank hits while carrying a ton of points. If you don't have Rhino or Nezha and you're new, I highly recommend letting other people pick up the points. Almost everyone will be playing Rhino anyway.
The other thing you need to know is that the enemies get slightly stronger every time someone enters the goal, so people won't want you to turn in very low amounts of points unless it would win the round or the time is about to expire. Usually people try to pick up 10-20 before making a goal.
everybody expects you to know what you are doing, which I do not, thus I never even touched it.
I was in the same position as you, but it seems like the game has been simplified so it's a lot harder to screw up. As long as you aren't constantly dying or turning in single-digit points, you're fine.