Talk about a broad question!
Errr.. budget? $20,000? $200?
When you've figured that out, start with looking at what you're scanning - is it just 35mm negs, medium format, prints, all of the above?
Then go to a review site on the web and compare models from the major manufacturers. There are important factors like resolution, image processing (dust & scratches, sharpening etc.), scan time, ease of use, compatibility (mac / PC / Linux)
If you can be more specific, someone will doubtless have first hand experience of a product and be able to help you further.
Good luck.
Rob