What you need
Your router, its power adapter, an Ethernet cable, your ISP modem/ONT if separate, and a phone or computer.
1. Connect the internet side
Connect the modem or ONT Ethernet output to the router's WAN/Internet port. Do not confuse a LAN port with the WAN port.
2. Power up and sign in
Power on the equipment and use the manufacturer's documented app or local setup address. Change the administrator password if the router asks you to create one.
3. Configure Wi-Fi
Choose a recognizable network name and a strong unique Wi-Fi password. Use WPA2 or WPA3 where supported and avoid obsolete security modes.
4. Place and test
Place the router in an open, reasonably central position rather than hidden behind furniture. Test several rooms before deciding that you need an extender or mesh system.