I'd recommend starting out with the Notes app that is built-in to iOS and determining what other functionality (if any) you would like - as well as if you can effectively use the on-screen keyboard.
While Notes Plus and Note Taker apps mentioned have a ton of functionality, they are, at their core, handwriting apps. This means that when you want to type, you have to select and insert a text box over the page (think text boxes in Excel). Notability is a more advanced typing-first application, which may be worth a try, although for ease of use, I still use the built-in Notes program for typing and uPad for handwriting.
I'd also recommend you go to the app store and filter for the "Productivity" category. You will see a number of typing apps as you go through the top 40 or 50 listed.
Finally, this thread may be helpful. While it focuses on both handwriting and typing apps, it was helpful for me.
http://www.ipadforums.net/ipad-apps/2037-best-note-taking-app-ipad.html