Where do I go to renew my subscription?
When you log into the game, the rightmost tab is "Store", where you can buy a Silver subscription (half the exapansions) or a Gold subscription (all the expansions) for time intervals from 1 month to 1 year.
If you want to see which expansions you are currently subscribed to (and when they will run out), you can do that in the "Account" tab (to the left of "Store").