2D Game Artist
Learn GIMP For Game Art

There's a tutorial on udemy for this exact thing. Gimp for 2d game art.

https://www.udemy.com/course/gimpforgames/

I haven't taken this particular course but can recommend the makers as I've tried their other courses

https://www.udemy.com/course/gimpforgames/

I have been making textures for about 3 and a half years now for my resource pack, Immersion. The following are my suggestions based off of that experience.

I would highly recommend using either Photoshop of GIMP (a free open source alternative to Photoshop), as they are much more powerful and provide a lot of more advanced features that will come in handy as you grow more experienced. From what I can tell, Paint.net can be easier for beginners, and can do a lot of the basic tasks, but I still wouldn't recommend it as it leaves less room for future growth. Granted, I have never used paint.net, so I am somewhat biased.

As to resources to learn texturing, that depends on what you're looking for. If you're looking to make 32x, 64x, or even possibly 128x textures, I'd recommend this tutorial: https://www.youtube.com/watch?v=xw65wUfEKIc. This was the video that helped me to grasp the basics of texturing when I first started out, and can be followed just as well in GIMP as in Photoshop. I also have started a semi-tutorial series of my own, in which I explain my general texturing process for my pack. My tutorials are very specific to my workflow, so much of it may not apply to your needs, but they still might be helpful to get the general idea of a texturing workflow. Here is a link to that, if you're interested: https://www.youtube.com/watch?v=gKHQyrJl2kw&list=PLfTIKSIL4Zbz4iKM5UQnY-a1-aWBs8U9G&index=1.

If you're only looking to make 16x textures, then some of the previous resources might be useful still, but much of it will be unnecessary. I admit that I am not very experienced with 16x texturing, or the workflow involved, so I cannot provide a lot of help for that. However, if you want a structured way to learn general pixel art, are dedicated to it, and have a little money to spare, there is an excellent online course by Gamedev.tv on it, which starts from an absolute beginner's level, so that virtually anyone can comprehend and follow along. I myself learned a few tips from the course, despite having been texturing for a couple of years already when I bought it. Here is the link to that: https://www.udemy.com/course/gimpforgames/. It's on sale right now for a couple of days, but if you miss that, just wait for another sale. You shouldn't be paying anything more than probably 10 to 15 dollars.

Now, if you want to make high res textures (256x and above), again I don't have a lot of insight to offer. I myself am trying to get into it, but it is notably different from (and harder than) lower resolutions. There are plenty of useful tutorials out there, if you're able/willing to use a photo as a base, but resources are currently extremely scarce for creating textures from scratch (which is what I want to do).

So that about sums up the resources I would recommend to get started. From there, it's just a matter of making textures, and occasionally experimenting with other tools and features in your chosen application. However, if you have any questions, feel free to ask. Also, if you want more feedback or suggestions for specific textures or general workflow, you're welcome to join my Quin Studios discord. I'm very active there, and can answer questions you might have along your journey. The Quin Studios community could also probably offer constructive criticism on textures, which can be very helpful to the learning process.

With all that said, hopefully this has been useful, and best of luck on your texturing journey! :)