Go to amazon.com
Click the "sign in" button
fill in your email and password and sign in
in the grey area at the top, click "your earnings summary"
top left you will see "signed in as............."
that's your affiliate code.
Yes you can use both Amazon and Google Adsense on your blogs - Adsense policies say that if using other ads on your sites, just to make sure that they look different from Adsense ads.
Google Adsense ads do look different from Amazon ads.