|I know that my internet connection is good because four other computers that we use in the house are fine. My i-Mac|
Are these other computers connected wirelessly, or are they plugged into the router?
If they're connected wirelessly then the issue is most likely with the one computer. I would suggest updating the drivers for the wireless interface as your first step. Try that and report back whether or not that fixes the issue.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.