iMac Connection Issue

[28Gauge]

I'm still a nube when it comes to Mac's since I have only had mine for 6 months. The one problem that I have is that my 2011 27" iMac will not automatically connect to my home wifi network when I start it up or wake it from sleep mode. I always have to manually select my network from the list of surrounding networks. My router is a 2011 Airport extreme with all the current software updates installed. Can someone help me figure this out. Is it a matter of a box not being check somewhere in the network preferences? Here is a screen shot if what I currently have selected.