Audition monologues! Watch
It depends on your strengths what style/era etc you like - have you read/studied/seen any plays you really liked? If not, go to a big public library and just look through some play texts. You might even be able to find anthologies of audition speeches.
Make sure its relevant to your age - trying to do a piece as a 70 year old isnt a sensible move. Dont attempt an unfamiliar accent unless you can actually do it well. And try to give a contrast to your other piece - ie. not both comedy.
Suggestions - Mary Mooney in "Once a Catholic", Eliza Doolittle in "Pygmalion", Sally in "Love on the Dole", Grusha in "The Caucasian Chalk Circle", Alison in 'Look Back in Anger'.
Avoid - Victoria Wood, Alan Bennett, Samuel Beckett, and anything ‘aggressive’