One common problem is that the WiFi switch may be turned off. Go to your phone's settings and make sure that it is on. Also make sure that there is in fact available WiFi in your area. If this does now work refer to the Troubleshooting Page
Sometimes all you need to do to fix this is to reboot it. This can be done by holding the power and volume buttons until the screen goes black, and then releasing the buttons. If this does not work refer to the other possible problems and solutions on the Troubleshooting Page
First off, make sure that your phone is not on mute. The volume button is on the right side of the phone. You can also try rebooting your phone. If this does not work, you may have a defective head phone jack and/or a defective speaker, and you can refer to the guides on how to replace these.