What is the best way to earn XP

Answer 1

as you are bronze
complete course without hint (more than 1000xp)
challenge other (since you get bonus answer if you lose, so nothing to lose)

Answer 2

Challenges, Lessons, Best Answer, Codes, Badges.

The one you are most suited with.

Answer 3

Submit Quizzes. Creator and Creator Master badges are worth 2000 XP and 5000 XP respectively.