My iphone 4 can't connect to Wi-Fi?

My iphone 4 will not connect to my home Wi-Fi. I type the correct pass word but it gives me the message "unable to join" or something like that.need help please

- Have you tried to restart your Wi-Fi router?
- Have you tried to restart your iPhone?
- Have you checked if your iPhone has AirPlane Mode off?

If 'yes' to all those questions, then we need to check if the problem is with your iPhone or your router.
- Are other iPhones there that can connect to the Wi-Fi router?
- Are you able to connect your iPhone to another Wi-Fi (friend, school, neighbour, etc.)?

Hope it helps,
Gunnar