Hi Maura:
I'm a bit of a frequent flier/points whore, so I'll share what I know.
Basically, you've got two options... you can pay cash for your coach ticket and use points to upgrade to first, or you can use points and to get first class seats.
If you use points to get your economy ticket, you won't be able to upgrade - period. You are at the bottom rung of their ticket codes and upgrades aren't allowed. Even when I had super-deluxe-platinum status, if I used points to get an econ ticket, I could never get an upgrade; not even when first was empty.
Either way, your best bet is to call the airline's frequent flier number and work with them directly. If you don't like the answer you get, call back and talk to a different agent 15 minutes later. Not all of the customer service reps are great with the customer service, but the ones that are will bend over backward to get you what you need. Sometimes, there aren't seats available, but call back every few days and see what opens up.
Good luck!!!