12/25 is a good all round riding cassette for most people
However, the only difference between a 12/25 and a 12/27 is that the lowest two gears on the latter are different. This makes it a good climbing cassette.
Since you are out of shape, on hills, and it doesn't sound like you intend to race, a racing cassette (11/23 or tighter) would probably not work well for you. What you want is some range between the top and the bottom number.
Do you have two rings or three in front? If you have 2, knowing nothing else about you, I'd really be inclined to recommend the 12/27. If you have 3, either the 12/25 or 12/27 should be fine.
To figure out what you have now, just count the number of teeth on the bottom and top cogs.