Welcome to the Serebii.net forums! We hope you enjoy your time here, and we look forward to seeing your posts!
As for which clan to join, consider these:
The Moonlight Heroes (one of the best I've ever seen), The Battling Academy (I question this one sometimes), or the Eevee Elite.
Also, for competitive battling, you can also sign up to challenge a League to test your skills against other experienced battlers. Try the Weather League for this one. We are currently seeking challengers.
If you have any other questions, feel free to PM me. I'll be glad to help you!