@Mvq sums it up short and precise. US seems dead, whereas as EU (at least for the time being) is rather active. We thrive towards having competitive games by sniping each other in pre-made groups. Up to 5 players on each side. We have even been experiencing 10 twinks on one team.
However, I am also going to be honest. You are about one expansion too late as of when twinks were at its peak in terms of competitive games. Therefore, the true competitiveness may lie at endgame. However, I still encourage you to make a twink! The community is great, and has a lot of helpful individuals that will set you up on any region, any faction. US might be back one day, and you could be ready for that by having a twink, playing the gulch, learn the differences between endgame and this bracket.
US "should" become active again when the Twink Cup is closing in (the annual tournament for twinks).
Before answering your concrete questions, I would like to go over a few classes that I think might be interesting to you.
Balance Druid: This is arguably the most versatile class you will find at the moment with a high chance of success overall. It has great potential in mid in terms of damage, and can take blow by soaking bursts in bear. It's also a very objective class because of its mobility (which is handy as an FC, and getting around the map overall), stealth, and 1 vs 1 potential. Needless to say, you can take on one and, or, every role possible within the gulch. Furthermore, you have regrowth for self-sustainability and off-heals.
Restoration Druid: You will mainly see this being the FC of the bracket, due to its mobility, even better self-sustainability than the balance druid, and still moderate damage in 1 vs 1 situations, not to mention the soaking potential with bear form as well.
Outlaw Rogue: One of the weaker classes at the moment, BUT! You can be a game-changing factor if played correctly! You add a lot of utility with your saps, cheap shot and kicks. Rogues used to be able to 1 vs 1 isolated targets. Rogue damage has been nerfed significantly, making 1 vs 1 situations near impossible as a rogue if the enemy is somewhat decent at the game. You will mostly be looking to set up objective kills on o with the help of some other classes, utilize your CC in mid, delay and possibly set up on a crossing EFC. I am not going to mention the other rogue specs, because they are an absolute joke at the moment.
Restoration Shaman: One of the top tier healers alongside with Discipline Priest. The reason I mention this class is because it has a great utility tool set next to having great heals. Restoration shamans have Ghost Wolf, making them easy to position themselves around the map, whether it is meeting with their FC, pulling back in mid, and what else you would have to do at a given moment. Restoration Shamans also have Earthbind Totem, which can be placed manually at an appointed location (it no longer just drops at your feet!). This means you can slow a lot in the enemy team in a mid fight, where movement is crucial, since positioning is almost "everything" in this bracket. Last but not least, you have sheer! Being able to stop incoming CC is a godsent as a healer. You can also stop heavy burst, such as a chaos bolt, or even an enemy heal, should you be in range. A lot of playmaking possibilities!
Discipline Priest: As mentioned under the Restoration Shaman section, it is one of the top tier healers alongside the Restoration Shaman. It has the possibility to do great healing, as well as dishing out some serious damage. The alternation between healing and damage is what makes this class great in midfield. Discipline Priests also has Power Word: Shield, which means they can grant stealthers a "re" when they are dotted. And that's great, considering the meta and amount of dots in the bracket. Power Word: Shield also serves as an "oh shit" button, allowing you to save a teammate or yourself to soak up some damage, and top up your teammate, or yourself. Never pre-shield! It's just gonna set the enemy team up for a burst! The shield absorb goes down to rot damage, and boom you no longer have a reactionary "oh shit" button for the situation that is currently playing out.
Now for your questions.
Faction: Make the class(es) that interests you the most on both factions. That way you can go with whatever faction that needs you the most. This makes for better competitive games.
Class: I believe the classes I mentioned before might be interesting to you. Considering the possibilities within each class. However, a TL;DR - play boomkin, rogue, rsham, or disc.
Gearing: Gearing is rather easy these days, considering how instanced PvP works. You want relevant armor type for your class. Meaning, leather for rogues, cloth for mages etc. Start out with full heirloom, Problem Solving Pendant (neck), The Rock x2 (finger), wand (main-hand for caster if you can equip one), Stratholme Lily (as offhand for casters). Some pieces will eventually get swapped out for the ilvl 27 pieces you get from the crates for winning a random BG. As for trinkets, the non-grandfathered BiS is fishing trinket from winning the Stranglethorn fishing extravaganza, or Arena Grand Master. Heirloom trinkets are a good start, or the minor recombobulator (ilvl 28) from engineering. Gearing for world PvP is different. I am sure you can find some old armories on here, or search for an old gearing guide. Just add legion neck enchant instead of "gift of x" enchant and you're good to go.
Location: EU is rather active at the moment, whereas US is rather dead. For EU, you want to be on Draenor, Aggramar, or Stormscale. I would recommend Draenor. For US, you want to be on Bleeding Hollow.
Guild/Community: EU has a lot of guilds currently. Any would do for a start. As for US, there's an official XPOFF guild for each faction on Bleeding Hollow.
Hope this was helpful to you.