How come my iMac gets slower Wifi than my friend's MacBook? How can I fix this?

The number of bars don’t indicate the speed of the conection just how strong the signal is that the wifi adapter is recieving.

To test the speed of the connection you can use a site like http://speedtest.net/.

Meters matter. If the Macbook was in more of an unblocked, direct line to the WiFi router, and if your iMac is located behind a wall or otherwise isolated, the reception on the iMac would be lower. Also, the Macbook Airport antenna might get better reception because it is near the outside of the case. The iMac antenna might be buried further within it’s case. I don’t know this for sure, though. I haven’t yet seen the position of the new iMac’s antenna.

@Lightlyseared is right. The amount of bars won’t affect network speed. You’ll either get the same speed or none at all.