how do i empty the trash on my mac book air to provide more space on the start up disk?

I am not very IT minded and when I start up my mac book air I get a message stating that the start up disk is full and that I need to delete files. Other users have advised to empty the trash. Is this simply a matter of clicking on the trash can icon and then the 'empty' button, or are there other places that trash is stored that I might not be aware of. There did not appear to be much in the trash that I could see but after deleting the items I am still getting the message that the start up disk is full.

Can anyone help as this is causing problems when trying to sync iphone?