NOTE: Most of these answers are based on information taken from the Patch 3.0.8 PTR, such as gear availability and ratings requirements. If the items don't show up when servers go live today, this post will be updated when they finally do.
At what level can I join Arenas?
You can enter Arena skirmishes at any level, with brackets at Level 71-75 and 76-80. However, only Level 70 and Level 80 players may participate in rated Arena matches, acquire personal and team ratings, and earn Arena Points to purchase Arena gear. Only Level 80 players are eligible for end-of-season rewards such as titles and possible new 310% speed flying mounts, although none have been announced or found in the data files as of this writing.
How do I form an Arena team?
The steps in forming an Arena team are very similar to forming a guild. You can visit any of the Goblin Arena Organizers in any of the capital cities, Blade's Edge Mountains, Nagrand, or Tanaris and purchase an Arena Team Charter. Unless prices have been inflated for Level 80, a 2v2 Team Charter should cost 80 Gold, a 3v3 Charter is 120 Gold, and a 5v5 Charter is 200 Gold. Level 80 players might find it easier to talk to "Baroness" Llana or Kanika Goldwell in the Underbelly of Dalaran to purchase their team charters.
Simply get the appropriate number of players to sign your team charter (2, 3, and 5 including yourself) and submit this to any Arena Organizer to register it. You will be asked to finalize your team name and choose your team colors, after which your team becomes eligible to participate in Arenas. Team Captains can invite more players into your Arena team by typing '/teaminvite [2v2, 3v3, 5v5] [player]'. An Arena team can have up to twice its bracket size in members (e.g., 4 members for a 2v2 team). You can read more about the basics of Arena forming in this Arena primer.
When does Season 5 start?
December 16, or 17 if you're based in Europe or Asia. That means you should probably head to the Underbelly and register your teams now.
How long will Season 5 last?
Blizzard hasn't announced it yet, although Arena Seasons tend to last for several months. Season one lasted for 20 weeks, Season two lasted for 23 weeks, while Season three was inordinately long and lasted 29 weeks. In contrast, Season 4 lasted a mere 17 weeks to accommodate Patch 3.0 and the eventual release of Wrath of the Lich King. Even though end-of-season rewards were calculated at the end of Season 4, rated matches and Arena Points continued to be awarded for a month after that.
Arena Seasons tend to keep pace with PvE progression, staying a couple of paces behind. With this philosophy in mind, it's not unreasonable to expect Arena Season 6 to follow sometime after Blizzard releases the next major content patch, which is supposedly Ulduar. Expect most Arena and Honor PvP gear to saturate the player base before the next season begins.
When and where can I buy my Arena PvP gear?
As long as you have the Arena Points and appropriate personal and team ratings, you can head over to any Arena Vendor in major cities and see what they have to offer. Level 80 Arena gear are available in Dalaran's Underbelly, with Zom Bocom, the Apprentice Arena Vendor, selling rare quality Savage Gladiator items for a low combination of Honor and Arena Points. Zaxi Smolderpipe, the Arena Vendor, sells epic Hateful Gladiator gear for more Honor and Arena Points, as well as requiring a moderate personal and team rating. Nargle Lashcord, the Veteran Arena Vendor, sells epic Deadly Gladiator gear purely for Arena Points and steep personal and team ratings. Level 70 Vengeful and Brutal Gladiator items will continue to be available through the Arena Vendors in Outland.
Can I use my Arena Points at Level 70 to purchase Level 80 Arena gear?
No. The Season 5 Arena items require Level 80 to purchase as well as personal and team ratings at the Level 80 bracket, which means players can't amass Arena Points and ratings at Level 70 and purchase items in Dalaran. Arena points, ratings, and teams are also wiped the moment a character turns Level 71.