I think you can search up clan tags in the in game clan section. But to do that, you can't currently be in a clan.
The extra premium account you won will be added on to your existing time. So if you had 15 days left and won 3 more days, it'll expire in 18 days.
Easiest way to earn xp in my opinion is to use Isida. It is best used with Viking or Hunter.
Customer support service by UserEcho