It takes 1. artistic skills 2. software for image creation. The better a sig looks(the majority of the time) the more time and effort is put into it. Also the ability and skills the artist has accquired over time effects the outcome of the sig also.
A person cannot accquire major artistic talens over night. I suggest you download GIMP and find tutorials out on the net for GIMP and get used to the tools. Play around with the program abit untill you get comfortable creating your own. Draw out what your wanting and then apply on trying to get the knowledge of creating that inside of GIMP to make your sig.