It is not easy to make a good gill section of a mushroom, but happily, for identification purposes, a small slice, squashed under the cover glass, is sufficient for identifying the special cells (cystides) that are important. And looking at the spores is easy: cut the stem from the cap and allow the cap to lie on a piece of paper, covered by a cup or something, for a few hours. The spores form a spore figure and can easily be put under the microscope.
Always take a fresh mushroom! Seeing the spores in an ascomycete is very easy: just cut a very small piece from the fruiting body and make a squash preparation; even with an objective 10x, (magnification 100x), the spores are easily visible, lying in a neat row of eight in the asci. Start looking at mushrooms with a good hand lens or a stereomicrocope. Try to find ascomycetes and look at the asci with the spores. For the spores and the special cells in gills an objective of 40x, often 100x is necessary.