The obvious bet is to get an HP printer, as their new ones support AirPrint - letting you print straight from the iPad to the printer.
http://www.hp.com/united-states/campaigns/airprint/index.html
There are applications you can find that will make any printer AirPrint compatible, but I wouldn't recommend them unless you're comfortable setting up unique software like that.
As far as which HP printer, depends what you want to use it for. If you need colour, you'll be spending a lot on ink. I threw out a colour printer because I didn't use it enough - when I did the ink cartridges had to be cleaned and refreshed so much it was a gigantic waste of money. I bought a cheap Samsung black and white laser printer and haven't looked back.