>>3>>2 While Paint Tool SAI does have a free download, it is still only like a 30 day trial. The full version is $50+ US. Most Wacom tablets should come with some kind of software to get you started though.
Also, if you do download the trial, any .sai images you have on your computer will become inaccessible after the trial ends so I highly recommend that you save them as .jpg or .png (despite the fact you can't access the layers after you do so).