Q: how can i download all my itunes purchases?

Depending on your MP3 player and what OS you use, you might want to check out these articles:

MP3 player compatibility with iTunes for Windows

http://support.apple.com/kb/HT2289

iTunes for Mac OS X: Compatible Players

http://support.apple.com/kb/HT2172

Also, this article can walk you through converting the song files to a couple of different formats, in case your music player doesn't support the default file type of iTunes (MPEG-4 AAC) :

http://support.apple.com/kb/HT1550

I hope this helps!  Best wishes and good luck!