As far as starting goes I would say block out what kinda tiles you need, what corners, what connections, and then go from there. You can always add new tiles to spice it up more later.
As far as making stuff look good and unique: practise, learning by doing. Which is what Pixelation is here for and which is why I am locking this thread and advise you to post you stuff in Pixel Art. There are also lots of examples of tilesets floating around on here if you use the search in the pixel art forum.