Wow

The PvP in WoW is a bit of a secondary feature, but can be good, at times. It can also be really bad at times as well, but that's speaking as someone who only occasionally dips a toe in. I think the main problem is that there aren't as many people who take the PvP side seriously, so if you're unlucky, you can end up in a battleground with a lot of people who just run off doing their own thing, rather than working together on the objectives, so they all just get picked off, and the other side romp to victory. But other times, you get in on a really good team, and the other side get the ones who don't play well with others.
World PvP is pretty much non-existent, from what I can gather, even on PvP servers. Most people can fly around in pretty much all the zones now, so you're rarely out in the open apart from at low levels, where you're not as likely to want to engage in PvP, and there's not as many people around your level anyway, you're more likely to get high levels picking off low level players. There are battlegrounds though, and there's arenas, which use smaller teams, and are more combat based, rather than objective.
As for getting started, buying the game will get you all of the expansions, bar the latest, meaning you can get to level 90 out of 100 just on that. Buying the current expansion gets you a level boost for either 90 or 100, I'm not sure if that one got upped to 100 as well as the boost you can buy in game... Even without boosting a character though, which I wouldn't recommend for your first anyway, it's pretty quick to get levelled now, for the most part. You can grind, but it is a lot slower, but questing has been streamlined a lot now, so you don't have as much wandering between places, you do quests around an area, then move on to the next, rather than keep going back and forth. Usually, you're high enough to move to the next zone long before you finish the quests in one zone, so if you did a few quests and some grinding in between, you could probably cut out the most tedious of the quests, and get through zones quick.
If you're thinking of joining a friend in the game, you can get them to invite you as a friend, which will give you a 1 week trial, and also link your accounts for 3 months, if I remember right, giving you a higher rate of exp when levelling together, and allowing you to port to each other. There are also several account perks for inviting someone who then subscribes themselves.
Finally, if you are buying the game, it's on offer if you buy direct from blizzard at the moment, at £3.50 for the base game, and £12 for the current expansion, Warlords of Draenor, and that price is going until the 4th of January.
If you want any more info on any part of the game, feel free to send me a message

 

You can play for free up to level 20, which is enough to get a rough idea of how a class works, but doesn't really reflect how they are at max level as you gain a lot more abilities, and in some cases, abilities get swapped out for others later on. I believe you can access one of the battlegrounds in the trial account but not 100% sure on that. If you know anyone else who plays, the recruit a friend feature is probably a better way to try it, as you get 7 days, with no level cap, and can level quick together.
As for the PvE side, it depends what you enjoy really. There's a lot of things to collect for those who like that sort of thing, a few raids for slightly larger scale coordinated PvE, some easy-mode raids for those who just want a bit of loot but don't want to worry much about boss tactics, and there's daily quests and 5 man dungeons too. The current expansion isn't that great though, and I'm only really playing because I can do so for free, and have a few friends who play. The PvP might be enjoyable as part of a PvP guild, but not having touched on that much, I can't really advise on that myself. As for how to play for free, you need to make a fair bit of gold per month, and use that to buy tokens you exchange for game time. Getting gold is fairly easy now, though it can be tedious, and to get it really easily, requires a few characters at max level. Lucky for me, I enjoy the levelling experience quite a bit, so I level quite a few different characters, and a few of those are just used to build up gold using their garrisons, which are a feature in the expansion at 90+.

 

Sadly Wow is a bit dead at the moment, simple fact is this is another period that blizzard has created known as a content gap.

 

So Horde then?