I was deleting my META.INF folder for Minecraft to install a mod, but I also deleted a bunch of other folders like my bin folder. How do I get my folders back?
To restore Minecraft files just run the game from the launcher. It will download any missing files.
I've found that to do that, I have to make a backup of every remaining folder, delete the entire .minecraft folder, then run the MC Launcher again. Then, restore your backup folders manually. I hope you didn't delete the saves folder, as then all your worlds would be lost. Unless you save your bin, you'll have to fix your settings too. And if you deleted your servers file when you go to multiplayer there will be no servers listed there.
If you did not emptied your recycle bin (is that whats its called in english? I dont know, edit me if I am wrong. Its the place where your deleted files go and leaves you a chance to restore them) on your computer. Normally, everything should be there in its folder, so you'll only have to take those folders back.
If you files are not there, you can simply launch Minecraft like you always do and it will download the missing files except your save files.
If your save files are also deleted and you cannot find them in your recycle bin, you can use a program like Recuva to restore your files if they have not yet been re-written. If you are not lucky, there might be some world corruption, but that would only happen if you waited a long time before using Recuva. I use Recuva myself and I can guarantee that it is malware free.