Use the search in this forum.
Check the sticker/label at the back of your modem. You will find the default ip address and username and password.
type in any browser 192.168.1.1 or 192.168.8.1 in newer versions then look for the profile management user name and password should be "admin"
Settings > WLAN Basic Settings > 802.11 authentication: "WPA" or "WPA2/PSK" > Create your 8 character password