Welcome to the site! Your profile picture is so cute! Btw your intro isn’t long at all, I’ve seen a lot of people introduce themselves like that!
To address the things you listed there, signatures are just pictures/words you can have under your posts, and a lot of people will also use it to give credit to whoever made their pfp or signature if they’re also using an image for it, and if they have a shop (like one where they’re selling things in New Horizons or one where they’re selling art), they might have a link to it there. But you don’t need to have one.
Carnival Coins are from an on site event that ended a while ago so they’re not useful anymore, but they were a currency that you could use to buy items specifically for that event.
Bells (most people just refer to them as tbt) are used to buy things in the shop, but the main thing they’re useful for are collectibles. They don’t really have any actual purpose besides sentimental value, but you can display them if you want, and a lot of people put them together in rows to show them off. If a collectible was bought at an earlier date, it will be displayed further to the right in relation to the other collectibles you’re displaying, and once the top row doesn’t have any space, it gets pushed to the bottom. If you want to get them in a specific order you have to consider the dates of your collectibles you’re displaying.
Oh, and seashells can be used as an alternative currency when buying certain things in the shop, like being able to upload gifs as your avatar.
I hope that wasn’t too confusing!
This will definitely help if you’re still confused on the site, and it explains this all way better than I ever could.