What are some "must have" apps for an android system. I'm starting off with the LG Optimus S. Any suggestions?
The first trick is to get the included apps for mail, contacts, calendar, and browser working to your satisfaction. If you're not happy with one or more of those, you can find replacements from the various messages on this forum.
Many of the places you'd normally access by browser have their own apps, e.g. Yahoo finance, your bank, amazon, amazon app store, weather sites, news and sports sites, etc. You'll want to try those.
There are lots of useful tools and widgets to do practical things and make your screen look the way you want.
And games, if you're so inclined.
One app you DON'T need is a task killer to kill apps "running" in the background. Search this forum to find out why.
This forum is your friend. Do some extensive reading here.