Some people are perfectly satisfied with observing commercially prepared slides. Others find much enjoyment in observing specimens of their immediate environment. In this case it is necessary to make temporary and/or permanent slides. Wet mounts (temporary slides) can be easily made by placing the specimen into a small droplet of water and then sandwiching it between slide and cover glass. This basic technique should be mastered by all microscopists. Other samples need to be first chemically processed, microtomes (cut into thin sections) before they can be mounted. This can be quite specimen specific and may require you to read some literature.
Commercially prepared permanent slides allow you to observe specimens which are otherwise difficult if not impossible to obtain. External parasites of animals (fleas), animal tissue sections, cell division stages, etc. are all interesting to look at. These are, however, difficult to make for inexperienced beginners, especially if the appropriate laboratory practice and materials are missing. Larger tissue parts have to be fixed (for preservation), embedded and then stained and microtomed (cut into thin sections). The laboratory work required may very well be beyond the capabilities of a beginner.
I therefore recommend the purchase of a box of permanent slides with interesting specimens for direct observation. School supplies companies do offer a selection of such slides.