Assuming you have a Windows computer, first make sure you can see the Desktop and no other app windows are open. Then make sure your phone is unlocked and connect it to your computer. You should hear a sound as the USB connection is made to your computer.
Most likely you will see a dialog box asking if you want to trust this computer. Click on yes.
You can also verify that your phone is connected to your computer by choosing Computer from the Start Menu. You should see see it listed at the bottom, after your hard disk, etc.
Then try opening iTunes and it should be recognized.