1. I think being successful on this game is a combination of two fundamental attributes, knowledge and skill.
You will need to know:
- Every class in the game inside out. This way you can anticipate an opponents next move in order to throw in a counter. As a Holy Paladin, I have an ability with makes me immune to silences for 6 seconds. Because I know Rogues, I know that if they use Vanish, it's either because they want to Garrote (silence) me from stealth. I've stopped myself and my partners from suffering a Garrote countless times this way. And it's only because I know Rogues inside out.
- The layout of the arenas. Know which points give LoS and which don't. Communicate positioning strategies with your partners. For example, on Blades Edge Arena, standing on the edge of the bridge you can stop a Rogue getting to your back.
- How to keybind. Keybindings are essential if you're going to get anywhere in PvP. Have every single one of your abilites bound to your keyboard and mouse buttons.
- How to customise your UI. For me, this played a huge part in getting Gladiator. Since starting Holy Paladin in S9, my UI changed somehow at least every week. Sometimes, a small configuration change to an existing addon and sometimes a whole new addon! Keep changing your UI until it's perfect for you. There are thousands of addons you can use!
- How MMR and the ladders work. There's a lot of information on here and WoW's site about this. Make sure you understand the MMR system and how the ladders work with regards to disqualifications and such.
- Use your keybinds effectively. Make sure you know all of your keybinds without fail. Make sure they're in an easy spot to reach aswell.
- Anticipate your opponents next move. This is the result of knowing all the classes. There's a good addon to help with anticipation called SpellAlerter. It alerts you of any spells that are cast by your enemies. You can choose which abilities it alerts you about.
- Communicate successfully with your partners. Keep calm whilst speaking on Skype. Make sure you don't rage or whine. It's important to keep morale up! If you lose a game, don't start blaming. Simply discuss a better strategy with your partners. Also, whilst in arena, make sure you only call out things that are necessary. Remember, whilst in arena, your partners aren't interested in when you are using Steady Shot or how much you hate Mages!
- Position yourself effectively. Positioning is key! Don't make yourself vulnerable by standing in LoS all day. Use LoS to your advantage and don't LoS/range your healer.
- Manage your cooldowns effectively. I see so many Holy Paladins who use Avenging Wrath and Divine Favor in the same macro. Hell, I used to do it too! Use your CDs well, don't waste but don't save!
3. Determination. Everyone has the potential to be great! You just have to dedicate your time and effort with a constant belief that you can do it! The more and harder you play, the better you'll become!
4. My first 2 months of PvP started in S9 (First season of Cataclysm). This was when I first decided to try and become great! I got fully keybinded and transferred to my friend's server where I played to 2066 as Rogue/Warrior/H.Paladin
5. That depends entirely on the player. Some people learn fast, others slow. Some are stupid, some are smart. I agree with what Atxx says about goals. This is a very important process of improvement. Don't start thinking about Gladiator just yet. Achieve the achievable first.