Using an iPad is a very different experience to the traditional Windows experience. Depending on what data you have, what applications you need to use to open it and also how well your internet connection works the answer may be different.
Cloud services will be key, most will offer upto 5GB for free (including Apple iCloud), and charge for more. This will provide a simple way to get your files and data onto your iPad, and then also add a point of replication so if you lose your iPad you still have your files somewhere.
I see that Donkichot has replied with some great application suggestions, recommend them all.
If you can, move your stuff into a cloud service, this will then allow you to get access to anything you need on any device, including an iPad.