Welcome to the PVP forum! It might just be your first time setting foot in here, but that's okay, because we all want the same things:
1) That sweet, sweet GARO armor
2) Those gorgeous metal ponies
3) To crack the skulls of our enemies
So if you've put off doing PVP because it's confusing/overwhelming, I'd like to help. I just started in November so I don't claim to know everything (far from it) but for first timers, this should help:
1) You have PVP-related abilities that you can allocate points to based on your character's PVP rank. IF you've never PVP'ed before, your character will start at 0. The rank goes to 50. As you win (or lose) fights you will gain both experience and Wolf Marks. As you gain new PVP levels, you can allocate these points to different abilities. You do this by going to the main menu and choosing PVP Profile from the option on the left.
You can hit "Default" and get all your points back and reassign them to another class and it costs you nothing in terms of gil or wolf marks or seals or anything. You can also save your point allocations if you want to switch classes quickly.
2) Take everything you know about your rotation in PVE content and chuck it out the window. Even some of the abilities you'd never use in PVE (I'm looking at you, one-ilm punch) can be very helpful in PVP. Some things, like Protect and Foresight (I think) don't work in PVP so they are useless. Stoneskin does work.
3) Healers have the shortest queue times in Feast. Followed by Melee DPS. Then tanks, then ranged DPS. Healers also have the hardest learning curve.
4) There are level 50 striking dummies at Wolves Den Pier that you can practice on. They are up the stairs from where the quartermasters are, at the top of the ship.
5) Your regular armor will probably be fine starting out, but you want to maximize secondary stats. Morale does nothing in Feast.
6) There's a Discord called Team Tryhard that's very helpful - you can join them here: https://discord.gg/fJRQ66X - they're all mostly PVP veterans and can give you good advice if you're just starting out.
7) PVPaissa.com is a good little site that has a basic primer on PVP if you're not sure why everyone's collecting medals, or what the boxes do in Feast or why you should care.
Hope this helps!