Mac users - viewing another imac?

A friend of mine who has an imac is helping me to get up to speed...given I am just coming over from pc. Is there any software you would recommend for iMacs that allow another imac user to log on and view your screen?

Messages works well and it's already on your iMac.

Messages (Mavericks): Share your screen

Thanks but when I go to messages and as I am messaging my friend the "share my screen with xxx" is greyed out? Does he have to be on his imac actively messaging me for that option?

It is important to understand a distinction ... screen sharing does not work via iMessages (texting). You both need to have an AIM, Jabber or Goggle Talk accounts (and the identities recorded in your contact information, I think). If you both have iCloud accounts setup then you already have all you need for an AIM connection ... just use <your user name>@icloud.com as an AIM account name.

Incidentally, Apple is rectifying this situation as of Yosemite (OS X 10.10, out hopefully soon). You will then be able to do screensharing without an AIM account.