Sorry you lost the nem, but you can chalk this up to to learning experience and grow from it. Take 'er slow, and do lots of reading. As Toddrex already said, your tank needs to cycle. I believe there are sticky's on the beginners forum all about tank cycling and when to add things. Damsels are generally a very hardy fish, that's why they're recommended for new tanks - they can withstand ammonia levels other fish can't. However, once the tank is running, most people don't like damsels anymore because though small, they're pretty nasty. After your tank has cycled, add things slowly. Each time you add something you'll go through another small cycle. Eventually when your tank is ready you can try adding another nem. Good luck, and ask lots of questions!
One more thing....nems can get sucked into even the tiniest of holes/cracks in overflows or powerheads. You can protect the powerheads by using plastic mesh to make a gaurd (found at craft stores for weaving ribbon through for bookmarks etc). Using zip ties make an appropriate shape to cover the inlet with a little space between the plastic mesh and the inlet. Fix it in place with superglue or more zip ties. Making your take safe for nems is almost like making a home "puppy-proof" with a new puppy. Make it safe and everyone will be happier.