Because I don't have the money (since I don't work yet), What free software or web pages you use to make your covers? 
And for those that had experience on it already, what makes a good cover? Something that people can look and say "hey I want to read that"

https://www.gimp.org/
http://www.openoffice.org/

Honestly, the most important thing for me is finding a good font. Readable, professional-looking, and fitting the feel of the book. You can slap any old stock image underneath and if the font and layout is good, it'll be good enough.

To go a step beyond good enough, you'd need to research further into what stories in your specific genre look like. There are conventions for covers that need to be taken into account. If you go to amazon and browse by category, it should quickly become obvious that a thriller will look very different from a romance, and so on. Pick as close a genre to your story as possible, look at a couple dozen covers, see what elements they have in common, and attempt to emulate that. For actual professional results, your only option may be to hire someone who knows what they're doing, but you can get decent results without needing to shell out too much.

At least on a site like this, pro level cover art isn't a requirement. Some people don't even bother with the title, just an image, and still make it to the top. In the end, the cover is only there to draw people to your story's page. Once they get there, it's the writing's job to keep them around. ;)

As far as software goes, I personally use clip studio, but it's not a free program. A lot of people use gimp as a free alternative, but I'm not sure how good it is myself. Photoshop is the classic, but even more expensive than clip. There are various websites that can be used for quick and dirty cover design, but I've personally always found their font selections insufficient and their layouts uninspiring.

Good luck!

Shutterstock can provide you with royalty free images. From there it's picking the best one for genre/feel of the book. I ditto editing it with GIMP as it's basically free photoshop(but it isn't as polished as photo shop, so you may have to look up a couple of details on how to do certain things)

I no longer have a PC, but when I did, the Gimp, Inkscape, and Blender were an unbeatable combination of tools at a ridiculously low price. In the Oughts I switched to the Gimp from Photoshop because the latter broke what was for me a few essential tools whose functions Adobe really should not have screwed with in their redesign. 

Now I use phone apps to edit pen and paper sketches.
Autodesk Sketchbook for coloration and detail work, Pluma SVG for vector graphics, and Snap Seed for photographic effects.

To be honestly, I use Clip Studio. Yes I know it's a paid program. But they do offer it half-off in the summer and Christmas. Worth saving up for if you do plenty of digital art and you like the way it works. I think the still offer a demo if you want to try it out.

However, before Clip Studio, of all the free digital art programs I used, I like FireAlpaca the best. It's simple and stupid easy to use. One might think it might be too basic, but sometimes basic is better.

Second Place goes to GIMP even though it's a bit much for digital art. Great if you are working for photos. However, if you like me and love digital paining, the tools don't seem right. I mean you can certainly just work in GIMP alone. I used GIMP longer than I did FireAlpaca, it just does too much when I needed a specialized tool. Not to mention GIMP is a little resource heavy.

So when I was using free, I had GIMP and FireAlpaca installed.

I agree with Asviloka about selecting a good font and matching the style of your genre.

If you're only posting here on Royal Road, the cover really doesn't have to be professional, but it does have to draw the readers' attention. I pay more attention to a story if the cover looks like an actual book cover (even if it's not professionally designed). At the other end of the spectrum, I'm not a big fan of, for example, a shadowy figure on a dark background, or anime figures that were obviously copied from someone else's art.

If you ever want to sell your books on Amazon, that's when you'll want a professionally designed cover, but at that point, it's a business expense for tax purposes.

Asviloka Wrote: As far as software goes, I personally use clip studio, but it's not a free program.

Clip Studio is free, right? I'm pretty sure I got emailed a code to unlock it, which still works even after 4 years.

I use mainly Krita (the free cousin of Photoshop).

As for source materials, I get my free stock images from pexels, pixabay or unsplash, and some of the pictures I take myself. As for free fonts, I recommend 1001fonts or the free stock of Peter Wiegel. 

Lol mine is an image I got off google with a title photoshopped underneath. Generally, the picture can either be a pull factor or neutral, but readers don't tend to weigh how good a cover is compared to other fics they might read, it's literally just to draw the eye. After that, your synopsis and rating (also reviews) are what will make them actually read your fic. The only bad cover image is no cover image