First of all I recommend you reading the New Player Guide I have recenty updated and posted here for some tips and general basic information about the game.
About gear and builds we are currently updating a Build Guide I hope to share here shortly. The issue with builds is that there is no “Best” build, it all depends how you play, what you like, your present state of progress and your goals… However, I can suggest you a pretty good build for early players that should help a lot and can be used with any class and spells:
If I had to recommend a class it would be mercenary, since it helps new players beating top content quite fast, incluing the top Kingdom Defense tiers. Here you have a proposed build for it:
Regarding joining a Kingdom, there are some recruiting posts here in the forums. Alternatively you can ask in the game chat to receive invitations. Best kingdoms are those with max tier Kingdom Defense and bazaar.
For more in-depth information I believe you already know where to go 