just listen to everyone else they know what to do ^^
in my case i open 2 paint aps and have a background on 1 and a character on the other
they i copy and paste the character onto the background with the second tool down on the select tool and thats about it ..... so just listen to the others for a better answer ^^;

yes practicing will get you better eventually that's how these great drawers are trust me I had to teach my friend to draw in real life I'm better on paper she wouldn't believe me but it was how I learnt and I finally taught her so with enough practice you'll get it in time it took me on paper to get good around 6 to 8 years besides some people actually do have awsome backgrounds that were even done on paint I didn't believe it utill I saw it so there's paint, sketcher (pixlr), gimp etc. and remember a good artist never does their work over night.

It takes a long time to develop, I'm still working on it. I became better at backgrounds through my tablet, cus a mouse hardly did anything for me. But its possible, after a lot of work and effort.
It can be done with MS paint, but it wont be as effective as photoshop. You just have to learn how to use different coloring in different areas. For example, if you were doing the sky, don't settle for a single blue shade, try mixing in other colors of blue to give it effect.

It isn't going to happen over night. To make these types of backgrounds you will need a tablet. A mouse would prove useless. Depends on how you want it to look really. But NO mouse could ever produce anything as nearly as good as what a tablet would do.

But yes, You will have to practice to become good at it.

Practice makes perfect.

And some people can draw really well in MS paint, YOU just have to practice.