How can I create and upload a logo on my Goo2o store?

One thing to remember is that if at any point you are going to have your logo on print material like Flyers or Banners, Business Cards etc, then it's better to have your logo designed in Illustrator so that it is a Vector image. It will be high quality and no matter how big your logo needs to be (even billboard size), it will never pixelate. Unfortunately with Photoshop is that it rasterizes images. Therefore if you had to increase the size to put it on a Billboard, you'd see all the little square pixels.