You might want to begin with 'Deadpool: The Merc with a Mouth'. It has a nice mix of comedy and adventure that showcases Deadpool's unique personality. Also, it sets the stage for some of his later storylines.
I'd recommend starting with 'Deadpool: Secret Invasion'. It's a great entry point and gives you a good sense of Deadpool's humor and action.
I think 'Deadpool Classic Vol. 1' is a good place to start. It features some of his early adventures and introduces key elements of his character and the world he inhabits.