Go on youtube, and first, watch some basic photoshop sig tuts. Practice and watch some more advanced.
The first thing that is wrong, is that the size is too big. Use 350x120
Then, a sig needs a good background, not just a pic. You need to also use gradient maps, brushes, c4d's, different layers, lightings, and better text fonts.
But all of the above is too much for a beginner, so watch some tuts first.