1. Alright I'm new to hacking, and basically have no idea what any of the terms mean. What does CWF, CIA etc. mean, and what do they entail?
CFW is Custom Firmware. The ones we have right now aren't permanent ones but only temporary. Meaning that you have to load them each time you power on the console. They patch certain regions of the console's system (mostly RAM patching) and allow you to do stuff like having region free, loading roms, installing not official stuff, etc...
CIA is like a digital archive and it works like eShop downloads. You put the CIA file in the SD card of the console and by using a CIA installer (Bigbluemenu, FBI, Devmenu, etc) you can install the games/apps/system files/etc that are inside the CIA archive. Depending on what you're installing, it gets stored in different places (either the console's NAND or the console's SD card). Might be simpler to use than to understand.
You might also hear about Sysnand. Sysnand is your console's real system or NAND (where the system files are stored). Every time you boot your console, you're using your sysnand. In order to use stuff like CFWs, your sysnand can't be above 9.2.
You might also hear about Emunand. Emunand is a copy of your console's real system or emulated NAND. It gets stored in a hidden partition of the console's SD card and has several purposes. One of them is being a safety layer since you can update it at will or install stuff in it and your real system will remain safe. Emunand can be updated to the latest firmware (10.1) when using a O3DS (Old 3DS) and up to 9.5 when using a N3DS (New 3DS). You can only have emunand if your real system has firmware 9.2 or below as well.
2. I have tubehax on my 3ds (both old and new with 9.0 on both) which CWF/CIA converter thingy should I use? (Pasta, Gateway, Sky etc.)
Honestly speaking, since both of your consoles have exploitable firmware, a free solution is always better. So for O3DS I recommend rxTools and for N3DS I recommend Reinand. You can find their threads here in the forum and you can also find guides on how to set them up in the forum's Tutorials section.
3. What exactly do the aforementioned things do? While I've read and watched tutorials on how to download them, they never really explained what they do. While I get the general idea is to install ROMs, how exactly do they do it, i.e. what do I need to have and do in order to install the ROMs?
I think I already explained in a general way this one before so I'll skip it. If someone else wants to explain better, be my guess.
4. How easy are these tools to use? most tutorials make it seem more complicated as I really have no idea what I'm doing.
Depends on your adaptability and on what you intend to do. But after reading a few guides and after asking a few questions, you should be able to handle it without much difficulty. It just feels like that now since you're new. Don't be afraid to ask questions.
5. So once I have something like Pasta on my 3ds what do I do then? Do I download ROMs? but then how do I load them on the 3ds itself? Also if someone could say what each program does that would be helpful as well
CFWs can only install CIAs. They can't use .3ds roms (the ones flashcart can use). So basically you install them the way I already explained above. For games, they'll appear in the home menu just like if you had downloaded them from eShop and you can play them normally.
Btw... Pasta CFW is discontinued and the team in charge of it is now working on rxTools. Some people still keep using Pasta but I would recommend choosing one that has an active team behind it, especially since Pasta doesn't even support emunand.
CakesFW, Reinand and rxTools are all options but like I said before, I recommend Reinand.
6. Can I download ROMs regardless of region? I have a US console, but can I play games from Japan or Europe?
Some CFWs have region free (at least rxTools has it for O3DS). Others don't but CIAs are be made region free (and mostly available like that already online). So no need to worry.
7. Also where can I find ROMs?
Google is your friend. You can't ask that here and we can't say either. You can however use the keywords "ISO" and "3DS" to search in Google.