Before playing Doubles, I honestly just played any singles tier I felt like. I played OU, UU, and NU mostly, and I was pretty damn bad at all of them (luckily not that bad anymore). Therefore, it was nice to find a favorite tier :)
When I first started playing Doubles, I had literally no idea what the fuck I was doing. I tried one VGC battle with a standard OU team because why tf not. Of course, I failed extremely badly, due to normal teams obviously not working effectively in Doubles at all. After that, I actually tried a little bit, found some super gimmicky ok strategies, and gradually became better and better at Doubles. Since then, I have become way better at Doubles than I will ever be in singles, simply because I am too used to Dubs.
I think the reason that I am so much better at Doubles is because of the fact that I think WAY too much in Singles. With the extra layer of prediction, it becomes easier for me to make safe predictions; there are few coin-flips in Doubles. This is what makes Doubles such a "whole new world" experience for newcomers, especially with Protect mindgames. It is something that both grabs newcomers and scares them away, but since I like to think so much, it caught onto me as soon as I began finding out how to play the game.
My beginner experience on the Doubles ladder was a very different perspective of Doubles than it is today. I went around using a completely retarded team, but since I knew how to use Protect and spread moves, I felt like I was frickin amazing at the tier (This is obviously because of how shitty the ladder is). This is a mistake, because once I started chatting on irc and posting in the Doubles thread of the OM forum, I realized how mediocre my strategies where. I mean srsly... Landorus-T and Heatran on the same team, as well as After You Cinccino and a Discharge spam duo. Fuckin idiot I was haha.
If I where to give any advice to people trying to get into Doubles after playing Singles a lot, the first tip is definitely to read about the basics. Aspects of the metagame that are different from Singles, such as Protect, spread moves, and excessive Speed control are all "basics"; you absolutely need to know how and why these strategies work before trying to get into Doubles seriously. Another thing to understand is this: There are attackers and supporters. No such thing as walls. It's frickin hard to counter Pokemon in Doubles, so try to overwhelm the opposition by setting Pokemon in check. Counters do not work very well at all, as the Pokemon you are trying to counter is able to attack your other Pokemon. A great tip that I myself found useful is this: Watch Replays of the pros, these replays will almost teach you the tier by themselves.
And also we definitely need Doubles tutors in gen 6 :>
That is all.