Try using the smallest hook (#10) you've got, normally found close to shore near weed beds and lily pads, target those areas. Dough balls will work, blood worms (from the Home shop) or the red worms from the compost pile (found in the small clearing at the end of the wooden bridge crossing) work also. Try a depth of about 60 cm (20 in). Fishing during the day during the higher parts of the wave indicated on the fish activity chart will help (from your Inventory select the Map tab then select the Weather Forecast tab to see this) use the time forward option (T on PC version).
GL.