When you say "shape of the map", do you mean you want higher resolution coastlines? If so, there are many places to download higher resolution data (see "Finding Geospatial Data" in the Mapping Toolbox help).
Alternatively, I find the borders.m function on the File Exchange extremely useful for this sort of thing.
worldmap('Fiji');
borders('Fiji', 'facecolor', [0.15 0.5 0.15]);