It’s covered in the FAQ on the General Discussion forum 
So essentially
- Visit the Forums on 50% of the last 100 Days
- Read 500 Topics created in the last 100 Days
- Read 20,000 Posts created in the last 100 Days
- Like 30 Posts
- Receive 20 Likes
- Reply to 10 different Topics
- Don’t have more than 5 Spam/Offensive Flags
- Don’t be currently suspended
in any Blizzard Game or Forum