To turn off PVP and turn it back on, you will need to head to a sanctuary in the game and press U to turn PVP on and off.
If you don't know what is sanctuary, or where to find one, In short, it's one of the major settlements that litter the game world's map, and In these locations, you will be able to turn PVP on and off.
However, before you can turn PVP on or off, we highly recommend leveling up a bit or joining a faction, as you will no doubt encounter higher-level players who will gank you.
If you want to avoid this, you can join a faction by speaking to one of the faction representatives in whatever settlement you are in or during the quest that forces you to do it.
To level up, all you need to do is farm quests, slay mobs, and gather tons of resources - it's going to take a bit, but this is the best way.
That being said, once you reached a good level and joined a strong faction, you will be ready for all matters of PVP fun, and can even accept missions from faction representatives in the game.
Check out some of our other guides for New World!