Check the wiki under creator resources for art (and art101.newgrounds.com). You'll find links to free apps you can download or use online. I didn't notice PaintTools SAI on the list (http://painttool-sai.en.softonic.com/). I've never used it but I think lots of art regs do.
If you have any other questions or want more suggestions/opinions, feel free to post in the art forum lounge.
Good luck :)
ReNaeNae
Hi Zach! Welcome to Newgrounds!
If you have questions about anything, just ask! :D
EaglescoutZach (Updated )
Thanks, the only thing i am having trouble with is submitting art stuff. I don't have any good art software and just use My Paint, so nothing i do on it can be submitted because of the format. Are there any good free art software?