You really need a laminated core for your transformer and you really need to use magnet wire. It is copper wire with a thin insulated coating. This book may be of some help Lindsay: Thordarson Transformer Manual
. Transformer Design & Construction
23853 is another book they have. Basically you use a ratio for your windings. if you input 120 volts into your primary and want 12 volts out use 10 times more windings on your primary than you do in the secondary also the wire is heavier on the secondary.