Suggestions:
1.) NEVER hold you breath in scuba... even if the regulator falls out of
your mouth, just blow bubbles.
2.) Breathing techniques... I use the Nike approach: just do it.
3.) If you are taking the Internet/CD based course, go find an instructor
who is willing to spend some time teaching you the finer points.  If you are
in a classroom setting, listen to what the instructor tells you.
4.) I assume that when you say you want to stay down longer, you are
referring to snorkeling.  If that is the case, your best bet is to keep
doing it and you'll get better.  You might also see if you can find someone
to teach you how to surface dive (although this should be covered in your
PADI course).