There are lots of programs to choose from, it depends on whether you want to purchase or are looking for free-ware or share-ware.
Skyscape.com has many of the published references like Taber's Cyclopedic dictionary, Davis' Drug Guide, and others available as downloads or even on memory expansion cards. They just released the 20th Ed. of Taber's and it has improvements over their last release including the appendices which I had found to be very useful in the book when I had bought it way back when.
Pepid.com offers an all-in-one suite that has a drug guide, an assortment of calculators (conversion, medication interactions, dosing), disease info, lab info, acls guidelines and more.
Do a search here for other threads about pdas, and a search online and you'll find a wealth of info. pdacortex.com and I believe the AACN website also have a lot of info about different programs or links to the different programs.
Whatever you decide have fun with your new tool - you'll be amazed at just how handy and indeispensible it will become in your day to day practice/life.