I actually just went through this last weekend!
I bought a game-time code from EA, and that has made me a "premium" aka "paid" member of TSO. You actually get the game when you download the free trial, so no need for additional software (Thanks to Parizad for answering that for me the other day!
)
When I first logged back in after having my 90-days credited, I could log in to TSO and create sims in the different villages, and yesterday prowled around EA Land. Before that, my only option was to log into TC3.
Here is the link for game codes-
http://www.tsogamecodes.com/store/index.asp
Once you've purchased it, you'll get an email on how to apply it to your account on the internet. The only problem I had was loading my game time code- I posted about that in another thread. It might be easier to do the monthly subscription thing.
Good luck- hope that helps!