Which cast on are you doing? You said two needle, but there are several different cast ons some of which use two needles.
Garter stitch is just knit ever stitch on every row. You put your right needle into the loop on the front from the left to right, wrap the yarn, pull through and push the old loop off the needle leaving you one stitch on your right needle. Do it again..and again. If you are doing the stitch correctly don't worry about how it looks. Just keep going and get the rhythm of knitting. Correct tension will come as you practice.
Did you watch the knit stitch video we have here? At the top of the page are videos - click on free videos/knit and choose the one that is appropriate.
There are also videos and still pictures at this site which might help, too.