Q: how to put more memory on desktop

iMac: How to remove or install memory

If you aren't comfortable doing this yourself, take your Mac to an Apple store or Apple certified repair provider.

First things first.   Although 2GB is the official memory level for Mountain Lion, 4GB is the minimum usually recommended in these columns.

Insofar as doing it yourself is concerned the best suppliers, Crucial in particular, provide their own advice with the packing slip.    As encouragement, there are probably few less competent than me but I managed to upgrade my memory with no trouble.