welcome to the forum!!
so basically TBT, also referred to as "bell tree bells" is the on-site currency. it can be used to buy stuff in-game, as well as collectibles and add-ons for your profile. you can earn TBT by posting on certain boards, but the main way to earn it is by trading with other users.
TBT is transferred by clicking on the amount of bells that the user has in their wallet, typing in an amount, and clicking "donate." you can also transfer TBT to another user by clicking on the "shop" button in the top-right corner of the screen and clicking "bells."
in-game items usually aren't worth too much, maybe about 5-10 tbt a piece, but the big money comes with collectibles, which can be as cheap as 10 tbt or as expensive as 12,000 tbt (when trading with other users, not buying them from the shop). a lot of common shop collectibles are worth less if you buy them from someone else (for example, the Ruby in the shop is worth 299 tbt but you can buy it from someone on her for about 150-200 tbt. also, in the shop the pear collectible is 39 tbt but since they're so common you could probably get it off of someone for about 10-15 tbt). but most collectibles aren't readily available in the shop (some are marked "sold out" and others aren't in the shop at all), so you have to get them from other users.
seashells are another form of on-site currency. 10 are earned upon joining, and ten more are earned after becoming a 2-year member. unlike tbt they can't be transferred between users. however, because the forum just recently transferred from vBulletin to XenForo the seashell system is a bit buggy so the staff has temporarily disbanded the use of seashells. they're likely fixing the issue right now and it will be back up and running in a short while.
there is a conversion of tbt to igb (in-game bells) and tbt to NMT but it changes constantly with the forum's economy. I don't personally know the current rates because I don't trade in-game stuff very much on here. you'll have to consult with people you're interested in buying from and ask them what their rates are, they tend to differ from person to person.
Hopefully that helps
