By pressing the power button on the back
The first iMac was produced in 1998.
I own an iMac and don't have that error. This should no happen and you should look into memory allocation before you next start it up.
There is no Start button per se. Finder is the equilivant of Start on a Windows PC.
This seems to be more of a technical problem. You should take your iMac into the nearest Apple Authorised Service Centre for help.
Click the link I will post for your answer.
No, you can only get that discount on a new iMac not a refurbished iMac.
There is no "iMac Air".
The original iMac was the G3 which came out in (1998). It was followed by the G4 (2002), then the G5 (2004), then the Intel iMac plastic version (2006). The 5th iMac was the aluminum iMac which came out in (August 2007), followed by the iMac Aluminum Unibody (2009). The newest iMac is simply called 2012 iMac and it came out on November 30, 2012.
Finder works just like the Start button on a Windows PC.
An iMac is a computer, but a computer is not necessarily an iMac, so they are not the same thing.
The iMac
The original iMac was created in 1998.