my sony ericsson phones screen is very dark and hard to see. what could be wrong...

Check battery. If its new, there shouldn't be anything wrong with it.
The only other thing I can suggest is that the backlight is set to a dark level.

Your back light may be set to 'dim.' Check under your settings. It should be under profile or power mode.
Other than that, have you removed the protective screen? Sometimes the plastic coating that they put on to keep it from getting scratched actually reduces visibility.