Describing people, especially in another language, requires some skills. First of all, to describe someone well, you need astute observation skills. To describe them well physically, you need to take note of physical characteristics. To describe their personality accurately, you need to spend a little time with them and converse at least enough to gather their mood, their interests or something else about them as a person.
When describing a person in French, you must keep the gender issue in mind. Every noun in French is classified as either masculine or feminine in gender. When you are speaking about people, the gender usually corresponds to the actual gender of the person being described. For example, if you are talking about "my father," "the boy," "he" or "that man," you will be using masculine nouns (or pronouns). And you must keep in mind that the adjectives you use to describe a person must be the same gender as the noun naming that person. So as you learn adjectives to describe people, be sure you memorize both the masculine and the feminine form.
Also, when you are describing people in French, you should be mindful of French culture if you are doing this in France. It is not rude to observe people in France; as a matter of fact, in France it may be acceptable for you to give someone an outright stare that would be considered very rude and intrusive in the United States. But conversely, in France, you need to be more careful about how loudly you speak as compared to the United States. For example, if you see a man with a long beard and want to comment on it to your friend, be sure that you speak in a very hushed tone of voice. The French usually speak very softly so that they cannot be heard by passersby. So while you should feel free to observe people, be careful how loudly you describe them if you are in France and out in public.