I suggest you use soft oil pastels, like Gallery brand soft oil pastels (not their regular oil pastels). Those are inexpensive. Then take a light color, say the lightest blue (stains less than colors with red or yellow in them). Use that light color to do your sketch. Then use darker colors where you want them.
If you're not satisfied with some of the lightly drawn sketch you make in the palest blue, you can first scrape it off (use an 80lb mixed media paper) with a single edge razor blade then erase with a mars plastic eraser. That will leave the paper almost clean or even 100% clean.
Some generally useful information about using colors: Hot colors, like yellow, orange, red, seem to come forward when viewed. Cool colors, like most greens and blues, seem to move back. So you would want the main figures/objects/abstract shapes in your painting to be warmer colors, as a general rule; and things you want to appear to be in the background would be painted with cooler colors. You get the idea.
Paint with a sense of fun and play. Be prepared to learn by experimenting. Enjoying it so that you stick with it. That's what will build your skills. It can be helpful to take a class with a good teacher, also a wonderful way to meet new people with similar interest.