you can get TBT by commenting on forums! for example, if you comment on something and take up 4 lines, you'll get 4 TBT! you can also get TBT by joining giveaways, selling things/villagers, etc.
you can buy stuff with TBT by first telling the person how much you're going to pay. second, at the top of the bell tree forums page (under blog tree, next to forum actions and quick links), click currency and the amount of bells you have will display. third, click the "change" under "to account" (i think that's what it's called) and the person you'd like to pay. fourth, put the amount you want to transfer and click proceed. you're done!
you can also click the amount of bells the person you're paying has (in their sidebar when commenting/about me page) and transfer there. this way is a lot easier. :>