On the forums, it's best to just have a look around and see what takes your fancy. There's a lot of them that have all their information well written out it's just a case of joining up!
In-game, check in Fluorite Plains around the stone bridge. That's a very common place to see people advertising their roleplays. Otherwise, looking at various movie clips that people put out is also a good way to get some information on roleplay groups. There's a lot of variety around both the forums and in-game, so it's hard to pick out the 'best' ones because of that.
I'd say just having a look around would be your best bet first and foremost!
Hope I helped, even just a little bit! If there's anything you're still unsure of, you need only ask!