Does this panel outside have one single main breaker that shuts off power to the entire panel and to the panel inside the house?
You can't just run regular NM-B romex to your shed, it needs to be UF romex which is rated for underground burial. And it needs to be buried 24" in the ground. And you'll need to install a peice of conduit with sweep fittings on each end to protect it, it can't just run exposed into the side of your shed. Or you'll need to install THHN/THWN individual wires in PVC conduit between the panel and your shed. This also is required to be buried a minimum 18".
You'll also need to contact your utilty company so all your utilites can be flagged prior to digging. Below is a link that'll show you how this circuit should be run to your shed.
http://popularmechanics.com/home_imp...er/index.phtml