But it can be answered rather short as follows:
1. Buy a wireless router.
2. Follow the instructions in the manual to connect it to the modem.
3. Follow the instructions in the manual to choose the ssid, the channel, the encryption and the key.
4. Ready.

#2 is easy: click in a cable
#3 is dependent on the make and model, so even an instruction video would end with "It can be different with your device".

Kees