So i'm very new to GOG, and PC gaming in the 21st Century lol. I've always used consoles.

I want to use MODs on Oblivion and Morrowind... but don't know where to start.

Do Nexus Mods work? How do I use them? To be honest, I really just want to hook up either my Xbox One or Dual Shock controller. Please don't try and convince me to use the keyboard and mouse... I'm just way more comfortable with my controller, plus health conditions make it easier for me to use. But wouldn't mind mods for other things too, once I get to grips with it.

Much appreciated :))))

I find it difficult to help. One reason is that I don't know what you have and how much you know. Which operating system is installed on your PC? If it's a Windows system, do you know how to use Windows Explorer and how to unpack compressed files (like ZIP files)? Do you know how to download files to your hard diks, and where to find the downloaded and the game files on your hard disk? Just a few examples.

And are you familiar with the unmodded games, and why do you want to play mods, and what kinds of mods? Have you made a selection already, or do you simply want to practise installing mods?

I will make two humbly suggestions.

For Morrowind, I suggest you to install OpenMW, a modern implementation of the game engine that address many issues with the original engine, enhance various aspects of the game, such as compatibility with modern systems and add support of Xinput controllers.
You must just follow the installation instructions: basically you have to download and install OpenMW, then you must simply import the game files into it.

For Oblivion, I suggest you to install this mod. You can watch this video for installation instructions.

Haha sorry I should've added more details. I'll elaborate.
Not a noob to PC's... just new to PC gaming. Ashamed to admit I'm a computer science grad.... who never liked PC gaming. Lets not go there.

I'm running on windows 10....

I'm using mods fine on Skyrim, but upon browsing Nexusmods for Oblivion, I came across one that apparently allows Xbox Controller to be used.. that's pretty much all I want. If things run fine, I'd probably play Vanilla for a bit and go back to Mods later... I'm still missing chunks of ES Lore, so don't want to run before I can walk so to speak. Thanks so much!!! I'll give them a try.

Good that I asked! :-)
In this case I can't help - I don't use controllers at all.

Regarding using mods in general, I've learnt over time to be more and more careful and reluctant. As you're certainly aware, the more you add to a system, the more likely that it breaks. And the value of any mod for you yourself is hard to predict. My recommendation is try out as many mods as you want (be prepared to sacrifice the time it requires!), but always ask yourself what benefit you expect from the mod, and if you get that benefit indeed (sometimes you may even get benefits you didn't expect :-)). Try to keep the number of mods running in parallel as small as possible (to avoid mod conflicts, technical ones as well as contentwise). And to become familiar with the base game before trying mods is highly advisable.

ES lore is a difficult topic, because to familiarise yourself with it is a time-consuming business. I suppose you know that uesp.net is a great reference site, and all TES game contain an abundance of TES lore. There are other sources of information available, but uesp + TES games is a solid basis.

Have fun!