1. Click the icon of your network manager in the top panel and select "Edit" or "Edit Connections".
2. Open the Wireless tab and select the wireless connection you want to use and click Edit.
3. Make sure the "Connect automatically" option is enabled, then click Save.
4. If you have more than one wireless connection, edit them and disable the "Connect automatically" option, or simply delete them all and leave only one wireless connection. That's it!