I need help on using oil pastels?

Oil pastels are difficult, as are oil paints. Obviously, the quality of the pastels is important. But because you likely need practice, you have the right idea of starting cheaply. Like oil paints and prismacolor colored pencils, oil pastels can be mixed to make shades of color and to produce a filled look. I would start by drawing larger size, more monochromatic objects. Perhaps try to draw an apple or a banana or something at a size of 6 or more inches just to get the feel for how to blend the colors in order to create shading. I say stay larger, because oil pastels are difficult to achieve detail with, in my opinion, particularly as you start out. In any case, I think that may be your best bet right now.