I'd hesitate to call any of the covenants noob-friendly, but you've got three main options:
Forest Hunters - Basically a PvP arena, you put on a ring and when someone enters the forest in human form you get called to automatically invade them. Lots of people hang out in the forest in human form to basically farm invaders, and there's no level limit so the people you're invading could be soul glitched level 700+ griefers. Probably not the best introduction to PvP, even though it's the first PvP-dedicated covenant you're likely to encounter.
Darkwraiths - Pretty much the go-to PvP covenant, it's what the overwhelming majority of invaders are. Use the stone, invade somebody at your level or higher in human form, rinse and repeat. You're pretty much always at a disadvantage unless you suppress your level and get kickin' fully-upgraded gear and invade low-level areas. Unsurprisingly this is what most invaders tend to do.
Blades of the Darkmoon - Probably the best recommendation I could make for a first PvP covenant, it's kind of a combination of the Forest Hunters and the Darkwraiths. You've got an invasion stone like the Darkwraiths which lets you invade any players who have sinned (which, due to the way sin is given out, is a shitload of people) with no lower level limit. In other words, you can invade people weaker than you without having to build your character from the word go with the intent of gaming the system. You also have a ring that summons you to fight human-form enemies in
, though that area also serves as a PvP campground similar to the forest. It's not too hard to alternate between stone and ring invasions as a Darkmoon, which is why I'm pretty fond of it.
The Path of the Dragon covenant is technically a PvP covenant, but it's basically a red soapstone that you need to be in a covenant to use. Non-covenant means of PvP, of course, are the red soapstone and good old-fashioned jolly co-operation. Since hosts need to be in human form both to summon co-op buddies and to be invaded you'll tend to get a lot of PvP action in co-op if you throw your sign down near bonfires. Red sign soapstone activity is most popular in the final area and the Painted World in my experience, though I once saw several red signs in the Tomb of Giants, which seems like a pretty masochistic place for dueling. I suppose it's no surprise that there's a contingent of Dark Souls fans who enjoy pain.