Typically you would start by creating a list containing all of the attributes you would like to see handled by the software. This is best achieved in a brainstorming session with representatives from all impacted business areas. A second session should prioritize those attributes into at least 'must have' and 'nice-to-have' categories. At that point, get the representatives to sign-off on your list in case they missed something and it comes back to bite you later.
Now you can investigate any available packages out there and generate a shortlist of no more than 5 vendors. Ask the various vendors to prepare a presentation or demo. Also ask them prior to their presentation to answer how their software handles each of the required attributes in writing and to focus their presentation on these aspects. This could come in handy later as a base for a scope document.
At the presentations get your business team back together and ask them to rate how each software package handles each attribute - usually a '1-5' approach is best. By adding up the points you can then move into the next round or decide on a package outright.
Hope this helps...