1- which is better: Draw on paper - Draw on ipad (using SketchBook Pro) - Draw on PC
Practice is the most important thing, and paper is generally the best for that. Buy a cheapo sketchbook and carry it around with you so you can draw whenever you have an idea. Most of your art should start out on paper, especially if you are relatively new. Always draw a couple different versions of something before making a choice. They don't have to be super detailed, just enough to give you and idea of what direction to go in.
iPad probably is not a good choice since I don't think the touch resolution is high enough to do very detailed work.
To get your art into a game, you will have to work on it on a PC desktop or laptop (or Mac) at some point. Either you draw a couple versions on paper, and then scan one of them in to get a digital copy (to then color), or you can redraw the sprite completely.
2- if on PC which program should i use ?
3- Which program is better for drawing ?
4- Which program is better for painting ?
5- what are the differences between PS and illustrator ?
These questions are all related, so I stuck them together. The most basic question is whether you want to try raster or vector. Raster art uses pixels, tiny squares, put together to form an image, and probably the more common way to make game art, but not always the best way. Photoshop is a raster editor, with GIMP being a free equivalent.
A subset of raster art is pixel art, which has a sort of fuzzy definition but basically means very small raster art that looks more "old school" like NES/Super NES games. Pixel art involves editting at a very small level and it's better to use more specialized software like GraphicsGale.
Vector images basically saves objects as a series of shapes, so a circle is a circle rather than a collection of squares like in raster. Flash games or something like Angry Birds uses vector graphics. The advantages of raster art include the fact that it's easier to animate and can be easier to work with, especially when making cartoony things. Illustrator is a vector editor, with Inkscape being a free equivalent.
TL;DR: Buy a sketchbook, and download Inkscape, GIMP, and GraphicsGale. They're free. Buy a book on drawing like this: http://www.amazon.co...rawing beginner, look up tutorials on Inkscape and GIMP (and GraphicsGale), and practice practice practice.