Begin by listing as many details about your subject as you can think of. From there, you can choose the strongest details and discard the rest. If your list is complete enough, you will have too many details to fit into one essay. Then group the details into logical (themed) subgroups that you can turn into paragraphs. Were I to describe my cat, I might arrange the details like this:

Body Paragraph 1: Physical Characteristics

Small, fine-bonedI remember that she is not still a kitten when I see the tiny bookcase opening into which she used to crawl.
Grey, with a white stripe from her nose running down her chin and then down her belly.
Medium length hair.
Body Paragraph 2: Personality

Alternately playful, cuddly, and anti-social.
Frequently prefers to be near, but not with, a person.
Does not like to be alone.
When she thinks she is alone, she will cuddle awhile with the first person she sees.
Loves to attack peoples hands and shoes.
Body Paragraph 3: Habits

Is definitely a creature of habit
To signal that she wishes to be taken outside or that she is not finished playing, she pounces on a persons calf.
Once something occurs in one place, it must occur only in that place from now on.
Because she was once played with underneath the living room table, she will run under the table when she sees someone take out one of her toys.
Has one specific spot in the living room where she likes to have her belly rubbed. Outside of that spot, she will bite the hand of a person who tries to rub her belly.