The best introductory issue of Deadpool is Deadpool Vol 1. #11.
Deadpool goes back in time to visit Spider-mans origin. Its perfect.
Even though its almost 20 years old, I think the Joe Kelly stuff is the way to go for newbies. Also, it doesnt look 20 years old.
Then id probably go Cable/Deadpool. After that either the first Way trade or first Duggan/Posehn.