I uploaded an image onto Krita, and sketched over it. Now I want to delete or hide the image so I can just work with the markings I made over it. Does anyone know how I can that?
I tried making it into a separate layer, but I can't quite figure out how.

You need to create a new paint layer for your sketches before you start sketching. Imagine putting a fresh sheet of tracing paper over a photograph or painting that you want to use as the basis of some artwork. You sketch on the tracing paper, not the photo or painting.

After you've opened your image, it will be on a layer, which you will be able to see on the layers docker. At the bottom left side of this layers docker there is a small '+' icon. Press that to create a new paint layer, which will appear above the image layer and be highlighted in blue to indicate that it is now the active layer. Your paint/draw actions will now take place in this layer so you can do your sketching there.

When your sketching is completed, you can click on the small 'eye' icon to the left of the image layer (in the layers docker) to make the image layer hidden. After this, you can add more layers as you require. It would be a good idea to start Krita and just play with the layers docker, making new layers, sketch in them, move them around, delete them, etc, etc. just to learn how they work.

I'd recommend that you read the documentation, which is useful and well written: https://docs.krita.org/Main_Page

Also, if you go to YouTube and search for 'Krita layers', you'll find many tutorial videos about this subject.