I have never used Photoshop. I have GIMP and Microsoft Paint and Paint.NET, and everything I need gets done in GIMP.
Yes, I think GIMP can open .psd files, though I've very little luck with that feature. Your mileage might vary though
As Bichur said, open up some skins and see how they're done. The best starters learn through trial and error.
Do you have anything in particular that you want to skin? Then download a skin from that app and open it up. Make a copy and start editing it. Then when you think you've done good, try that skin and see if it's good, or if you can improve it anywhere. See if it's distorted or not.
how to you use transparent pixels. do you need a transparent alpha channel.
A. if so how to i do it in GIMP
There are many ways to do that, but I use the Color To Aplha feature the most.
In GIMP, for example if you make an icon [Just suppose] and it's color is black on a white background. If you want to remove the white background from the icon, you select the layer with the white and go to Color> Color to Alpha. From there, choose the white color [they differ a bit in shade] and click OK.
I'd like to know about other ways to do this, it'd really help me a lot in GIMP