When you need to delete unwanted files or folders, move them to the Trash and empty it.

This is what the Trash looks like when it's empty.

This is what the Trash looks like when it's got something in it.

You can drag things to the Trash and then open it to see what's inside. However, when you tell the Trash to empty itself, the items are deleted. To get rid of unwanted items, simply drag the item from the Finder and drop it onto the Trash icon in the Dock. The item will remain in the Trash folder (click the Trash icon to view its contents) until you either move it out of the Trash (if you decide to keep it) or empty it. From the Finder menu, choose

Empty Trash.

If you're deleting sensitive files, such as electronic banking statements, tax returns, documents that contain social security numbers or passwords, you can choose to have your Mac securely empty the Trash: From the Finder menu, choose Secure Empty Trash. This makes your Mac write over your deleted files, greatly reducing the chances it could ever be recovered. This process can take some time, depending on what you're deleting.