The iPad isn't really the best thing in the world to draw on but if you still intend to draw on your iPad, Paper is a must. It's an absolutely beautiful iPad app with which you can sketch, draw, make notes and much more. The UI is super simple and intuitive, mainly based upon gestures and actions, instead of buttons. You can organise your drawings into books and share them instantly on to your favourite social networks.
The app itself is free, but to get the best use out of it you will need to buy the "essentials" package which is £5.49. The makers of the app also recommend getting a stylus so you can be a lot more accurate when drawing.