Officially confused on how to Subscribe

[Niwashi]

Yes, you have to buy the game in order to play it. Purchase of the game includes 30 days of subscription access, after which you need to renew that subscription to continue playing. (The $15 per month you saw was the price for renewing the subscription.)

EDIT:
Oh, you might be able to purchase the game for less than the standard 29.99, though, if you shop around a little. I think Amazon had it for around $20 the last time I checked. Just make sure you buy the right version for the platform you're using. (A PS3 copy wouldn't let you play on PC, and so on.) Given that purchase comes with your first month of access, it may not cost much more than the sub would anyway if you can find a good deal.