Your camera can focus as close as 20cm (approximately 8 inches) so make sure you have the lens no closer than 8" to whatever you want to focus on. Some cameras can focus at their minimum (8" in your case) when they are zoomed all the way in and some can only focus close when they are zoomed all the way out [full wide]-- not sure about your camera so you'll have to experiment. Try zooming all the way in and point the camera at something that has a lot of contrast and texture- the shell of a clam or a piece of liverock etc.-- hold the shutter release button halfway down til the autofocus gets a lock. If it can't focus then zoom out until it can get a focus lock, and then take your picture.
The bad news is that even zoomed all the way in your camera will only get so-so close-ups because the minimum focus distance [8"] is not that great and the zoom lens is not very long [96mm].
Hope this helps,
Greg