Need clarification..
One printer: What kind? Need manufacturer and model. Is it shared by several computers today? How?
Airport is 802.11 just wifi. So to "get Airport going" you would merely turn that option on in the Mac and find the base station - as you would with any other computer to use the wireless connection. (I hope you have already secured that wireless connection - if not, do that before adding any more computers and get whatever trojans, viruses and any other spyware/malware off any computers on that network before adding any computers - Macintosh or otherwise.)
If your printer is a print-only USB (and not ethernet) device, there are USB-to-wifi and USB to ethernet adapters available. If your printer is a multifunction, USB-based, printer/scanner/copier/fax, you will have problems sharing that over any network.
Worst case: If your printer is a print-only USB (and not ethernet) device, there are manual USB switches available... The printer stays connected all the time, so do all the computers... but you need to push the button for the corresponding computer to connect to the printer... I use one of those today to connect a Canon MP780 and share between two Macs and a HP laptop...
I plan on buying my MBP soon, but I only have one printer with two computers. What excatly do I need to get Airport going. Do I need the base station or just the Airport Express Base. I have wireless internet (DSL I think) and would like to not have to plug and unplug my printer when I need to print things. Thanks

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