iMac not conecting to a network

[LilBabiNothing]

I have a iMac in the last few weeks it will not conect to the network, sometime it says seaching for network and other times it's says it conected but the ineternet dose not work and other times it runs slow. The iPhone and other computers are fine on the network.

 

[Plymouthbreezer]

In order for anyone to help you, we need far more information...

I'm guessing it's a wireless network?