you ideally need to use at least f8 and keep a shutter speed of at least 1/160 or thereabouts. You can therefore either choose aperture priority (f8) or manual. Also, select macro AF or MF. Either way, you lock the focus and then rock back and forth until the subject is in focus and then squeeze the shutter. The depth of field is very shallow so it takes a bit or practice. I would avoid full zoom for a while and work at shorter focal lengths so the magnification isn't so great. When you've mastered that you can then practice at full zoom.
Oh, and you'll find a bit of fill-flash can help but you will need to diffuse the flash. If you're using the on-board flash try covering it with a tissue or similar (e.g. with a rubber band around it).
I hope that helps.
Matt