Well once you have joined click ABD (bank) then collect interest and you can spend it in the TBT Market place or get AC bells off people who collect TBT bells. (switch)
You can buy and sell virtual items or the like via the Bell System. You can collect interest via the bank. You give bells to another member by clicking Give Bells and typing in that User's ID. The TBT Shop isn't open yet, so don't get rid of your bells quite yet. Once the shop opens you will be able to start purchasing more with them. Right now they are sold for AC Bells by many. Others hold events using TBT bells. Like I said, more will be available with it when the store is opened.