The first question is what was your gearing on your old bike? If you want a low gear that's nearly as low as your old one, you need to know what it was.

Sheldon Brown's gearing chart is pretty limited, but it's easy enough to do your own spreadsheet, or someone probably has a more detailed one online. Figure out what your old gearing was. Ask your LBS what range of chainrings and cassettes you can put on without changing your cranks or derailleur(s). Figure out what gear ranges you'd get from different combinations of chainrings and cassettes.

(Nowadays you don't normally swap single cogs in the rear, that's why it's called a "cassette.")