There are three types of microscopes, optical, electron and scanning. I assume you are referring to optical microscopes.
Optical microscopes include dissecting and compound types. Dissecting scopes make use of magnifying lens to examine an intact specimen (either inorganic or biological), said specimen being lighted by a beam of direct light as reflected off of the specimen surface.
With a compound microscope (to which I assume your question applies), a thin slice or section of the specimen is examined with light transmitted through the specimen from underneath. In other words there is a thin slice of tissue (usually on a glass slide) on the stage of the microscope. The microscope consists of ocular lenses and objective lenses above the specimen, condenser lenses beneath the specimen, and a light source under the condenser. The specimen slice is usually stained with various dyes to make microstructure visible.
Your question is somewhat difficult to answer without knowing more information about what type of specimens you will be examining.
In general, however, I would say that your description of the microscopic image should include the structures or shapes or other details that allow you to identify the specimen and possibly any abnormalities or diseases that it may have.
So an adequate description of a flea, as examined microscopically, would include its overall appearance as in insect, the number of legs present, the bodily divisions, the antennae, and other items that define it as a flea as opposed to other insects.
Another different type of specimen such as a human skin biopsy showing common basal cell skin cancer would be described as skin with a configuration consistent with origin in a certain body area, in which there is a proliferation of basaloid cells in the epidermis with pallisading at the base, and invasion into the sub-adjacent dermis. The dermis could be described as showing actinic injury (sun damage). You should then describe the margins (sides and deep margin) as being free of or involved by tumor.
In summary, a proper microscopic description gives details that allow you (and any other expert as well) to identify the specimen and its abnormalities or disease. You should be brief and avoid details that have no bearing on the identification or diagnosis. And be mindful that a consultant who wishes to provide a second opinion on your diagnosis will request the slide and examine it herself, rather than relying solely on your microscopic description.