Why am I asked to join a WiFi network every time i open my computer?
Make sure your Mac is up to date with all Apple Software Updates.
Go to System Preferences > Network. Select Wi-Fi or AirPort from the left column. Click the Advanced... button.
Under the Preferred Networks list, select every Wi-Fi network that appears, including the one you want to use, and delete every one using the "–" (minus) button. Select the box "Remember networks this computer has joined" if it is not already selected.
OK then Apply.
Select your wireless network in your Mac's Wi-Fi menu, provide your wireless network password, and determine if that solves its connection problems.
If the above does not resolve the problem, follow the steps in these Apple Support documents:
OS X: Wi-Fi: How to troubleshoot Wi-Fi connectivity