Low Pressure From Shower Head Troubleshooting
Start with the water pressure in general - is the pressure OK when you're filling the tub?  How about the sinks?

Assuming the pressure is OK when filling the tub, that means there's probably a blockage, either in the diverter, or in the shower head itself.  Try taking the shower head off - how's the flow?  Try replacing the shower head - it could be the old one is clogged (you can get a cheap shower head for a few bucks, and if the one you have is decent quality try soaking it in some CLR).