A monologue is a part of a screenplay or a poem where a single character speaks his thoughts aloud to the audience or to another character. Monologues can be as short as 10 lines or as long as several pages, depending on the context. The monologue you choose depends on your strengths as an actor and on the requirements of your project.
Performing a poem, such as "My Last Duchess" by Robert Browning, as a dramatic monologue can be effective and powerful. "My Last Duchess" is the story of a duke's murder of his wife. The duke, who is describing the events, attempts to portray himself as being noble while, instead, he appears to be controlling and jealous. This monologue, which is tailored to male actors, demonstrates a revelation of character for the duke, a revelation that provides the actor who is performing it with the opportunity to interpret the character and perform it compellingly.
You can look to performing excerpts of plays, stories or poems, such as a part of "Eve's Diary" by Mark Twain. "Eve's Diary" is the alleged diary of Eve from the Biblical tale of Adam and Eve. What makes it funny is the dryness of the wit. Eve spends her time surveying the world around her, a world that is entirely new, and complaining about Adam. Because the work is so long, you can find any number of excerpts that will suit your length and time requirements and that will suit your comedic acting style.
Shakespeare's plays are full of monologues spoken by both men and women. Because Shakespeare's plays are classic, new and bold interpretations are both welcomed and advised against. Many directors already have an impression in mind of who Shakespeare's characters are, so any divergence may be read as being too strange for mainstream theater. Famous monologues for men include several soliloquies spoken by Hamlet in "Hamlet"; famous ones for women include several spoken by Imogen in "Cymbeline."
You can create your own monologue for an audition or a classroom assignment, depending on your abilities as a writer and the requirements to which you are working. Monologues should feature all of the components of a full-length play, including a problem, a point of attack, a climax and a denouement. If you're writing a comedic monologue, try not to be overtly comedic; attempt to express humor in concepts and ideas rather than in the words that you're using.