Hey, thanks for the reply.
I have already done this procedure. After this procedure, when I type iwconfig in terminal, it shows my WiFi in wlan0 (meaning, it got connected). Also, when I type ifconfig, it shows an IP Address in wlan0. But in right-top corner tab of my desktop, it still shows "WiFi Networks device not ready".
(edit) Now, I can access Internet through WiFi from RPi3, but SSH still does not work.