If you don't want your world to be similar to earth, why don't you just go random? In photoshop, you can use two simple effect to get a number of random-generated coastlines.
Select defualt colors (black as foreground, white as background) and Filter > Rendering > Clouds.
Then go Filter > Stylize > Trace Contour.
Now you have a lot of random generated landmass. You can copy/paste the shapes that you like the most and make a sort of collage to determine the shape of your continents. Some editing may be required to cut some parts to obtain "island" shapes surrounded by water.
This is what I got with a 1000x1000 image. If you use a larger map, you will have more shapes to choose from!
Happy mapmaking! =)