How to PvP in Dark Souls 3?

The fastest way to start PvP is to reach the second bonfire at the High Wall of Lothric, then return to Firelink Shrine. When you return, there should be an NPC named Leonhard leaning against Lothric's throne. He will give you Cracked Red Eye Orbs, which are one-time-use items that allow you to invade another player's world. You can get more of these items from the dark wraiths near the Farron Keep Perimeter. If you continue to follow Leonhard's questline, you'll eventually get a Red Eye Orb, which can be used an infinite number of times to invade.

You can also get the Red Sign Soapstone from a spell-casting worm outside of the Rosaria's Bedchamber bonfire (in Cathedral of the Deep), which allows you to be summoned for PvP in other people's worlds. If you see a summon sign that is red, it means that someone else used the Red Sign Soapstone -- if you interact with the red summon sign, you can summon them into your world for PvP.

There are also covenants you can join that will auto-summon you to other people's worlds for PvP. These covenants include the Watchdogs of Farron, Blue Sentinels, Aldritch Faithful, and Blades of the Darkmoon.

Here is a great Reddit thread that may help you:

So before you can even start to PvP, you need to know where everybody is scrappin'. The most popular location at the current moment is right outside of the Pontiff Sulyvahn Bonfire in Irithyll of the Boreal Valley. Immediately past the bonfire on that little balcony/open area you will see tons of summon signs. It is a great place both for spectating and fighting. Just place your sign there or summon other phantoms.

In addition, here are some guidelines for beginners:

Due to the fact that multiplayer is kind of like a side feature, it is not extremely fleshed out. There is no arena or official place to actually do strictly PvP. So, because of this, the community has made up a set of guide lines and rules that are sort of secretly understood among players. If you are doing invasions, anything goes. You do whatever you want or need to win, but when you are being summoned from a Red Sign Soapstone or you summon a phantom, they are expecting a duel with some small set rules. Acknowledge your opponent. This isn't really necessary, but more of a courtesy and etiquette. Do a wave gesture, bow, beckon. Hell, even throwing down a hello carving is fine. If your opponent is gesturing do not attack them until they are ready to go, it's rather rude. Now, there really are not a lot of rules. The biggest one is really 'no healing.' Some people might see this as silly but healing doesn't necessarily help you get better as a player. You play a lot sloppy knowing you have free heals. Plus, if both players are healing, the host has a huge advantage because they have double the estus you do. Your estus is halved when you are summoned to another world or invade. Plus, it would make duels just last way too long. This is especially annoying in fight clubs when other people are in line for a round in the ring. Also, if you are losing don't turn tail and try to book it to the boss or something like that. Just take your loss and learn from the mistakes.

The first DLC, Ashes of Ariandel, introduced dedicated PvP arenas to the game: Undead Match.

Undead Match is accessible from the bonfire in Firelink Shrine, but you need to acquire Champion's Bones and burn them in that bonfire first.

Champion's Bones are obtainable in the following DLC locations: either from killing the Champion's Gravetender & Gravetender Greatwolf boss in Painted World of Ariandel or after completing Lapp's quest line in the Ringed City DLC (but only if you don't own the first DLC).