'''96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '12 Surly Pacer, All are 3x8,9 or 10. It is hilly around here!
Definitely need more information. What type of bike, road, MTB, hybrid? What type of rear hub, freewheel or freehub? What's the frame's rear dropout spacing, 126, 130, 135? What make and model of derailleurs and shifters do you have?
The change can be relatively inexpensive or dreadfully costly depending on what you now have.
You will need a new rear wheel with a freehub for 8 or 9 speed cassette, an 8 speed cassette and new rear shifter. New shift cables and chain probably needed, but rear derailler will not be needed unless you are making big change to spread between top and bottom gears, or it is showing signs of wear.