Well you get them from posting (not in every board though). But if you didn't post and just got 5 bells out of nowhere then ye you can check your transaction log to see what happened. http://www.belltreeforums.com/vbshop.php?do=transactionlog
~Posting in threads in certain sections of the forum that you can earn them in
~Starting threads
~Voting in polls
Those are the easiest way to earn bells without partaking in the economy and market. You can send and recieve bells with other people, so you can also earn them from:
~Events and giveaways
~Buying and selling art, in-game stuff, and forum stuff
The 5 TBT you earned were probably from posting; the 6 more you earned that set you to 11 is probably from posting this thread. Hope this helps!
Be sure you're writing quality posts. One word replies can earn you an infraction. Also, the longer the post, the more TBT you'll earn. I believe you can earn up to 10 TBT per post.
*posting comments and also threads. the longer the post, the more bells you earn. but some sections don't earn you bells, like the TBT Marketplace for example. also, if you type out a long post but then edit it and delete everything, your bells that you earned from posting will be taken away.
*selling stuff: villagers, items in ACNL, digital or traditional art
*also by reselling forum collectibles (which are bought in the shop) that are restocked periodically or are available during events. there's a limited number so people buy them up and resell them for sometimes over 10x what they paid.