The first difference is the courts.

line 2: outdoor games => outdoor sports
line 4: (the size of) the ball
line 6: are not allowed to use => mustn't use

And volleyball players have to stay on their own side.

There is a net in middle of a volleyball court, but there is nothing blocking a basketball court.

But basketball players can run up and down the entire court freely.

You should leave out all personal material and also any irrelevent details about the items themselves--details that have nothing to do with the division of items into classes.

The second difference is the size of the ball and how hard it is.

It would be nice to start off with an Introduction--a short paragraph that will introduce the topic and inform the readers how many classes we are going to divide the items into.

Volleyballs are made of leather.
Basketballs are made of synthetic rubber and have a lot of dents all over the surface.

First, these two sports are different in the way they use the ball.

As you know from your many years of experience as students, You might also imagine that when schools divide students into classes they like to avoid unbalanced classes sizes, particularly classes of only one or two students.

Both are ball games and team sports, yet there are several differences between them.

The second difference is in the number of players.

These things might be The first two steps in writing a Classification Essay are to figure out (1) the topic--what kind of items to divide up and (2) how to divide the items.

A soccer team, on the other hand, has eleven.

The third difference is in the positions.