If you need to prepare your own piece for the casting calls you attend, you should choose a piece that shows the depth and breadth of your skills and indicates some kind of relation between your current skill set and the character you wish to portray.
Audition monologues can be drawn from a number of different sources. You can choose a well-known or a more obscure script. In addition, it is sometimes appropriate to choose a passage from a well-known or representational literary work to use in place of a scripted piece. It is usually not a good idea to write your own material, as the entire goal of acting is to bring someone else's work to life. Avoid famous material, since you do not want the auditor (casting director) to be comparing you to others who did the same work. With less famous material you may select to edit the material, including other characters out or material that is not central to the dramatic curve of the work.
Acting monologues should be memorized and well rehearsed to show that you can do more than expressively bring the material to life.
You might even consider hiring an acting coach or attending acting workshops to help you prepare for the audition. It is best for actors to select one coach or workshop and stay with it for at least six months, and often for years at a time (if the workshops or coaching provides what you need and over time personal and professional growth occurs).