If you are starting completely from scratch here you might want to invest in a book that explains some of the basics of navigation. The RYA Powerboat book is worth reading http://www.rya.org.uk/shop/products/Pages/g13.aspx
Nautical charts (which are the marine equivalent of a land map) show the sea and coastline, with depths, obstacles and navigation marks all identified. You can get these for large areas or just local bits of coast/estuary at different scales. Your local chandlers will have lots of them!
Or as others have suggested, you could invest in a GPS chartplotter, also available from your chandlers. These are like a car sat-nav, but they show a marine chart rather than a road map, with you boat's position plotted on the chart as you cruise around. A bit pricey but VERY useful.