Simply tell your mechanic you want more gears for climbing. She should be able to give you a number of options, depending on what you've got on your bike now.
Here are the two most common options. The second one is usually cheaper.
Add smaller chainrings in the front. If you've got a double, go to a compact or a triple. If you've got a road double, change to a mtn double. Any of these options might also mean changing your shifter, front derailleur, and/or bottom bracket.
Change to a cassette with a larger cog. Depending on how large you go, this might also mean changing your rear derailleur.