Do I need a app to use my pencil

You need apps to do everything with your iPad. If you mean do you need to install anything for the pencil, no you do not. You just follow the directions to pair your pencil to your iPad and it will work. Certain apps will use the pencil better than others, but it will work for most of your daily clicking and writing. The best thing I can suggest to give you ideas about what you can do with the pencil would be to check out some videos online to get some ideas.

What did you spend $100-$125 for an Pencil for?

What do you want to be able to do with an Pencil?