You don't need Visual Studio to edit the layout.master. You can edit it with any text editor.
If you want to create your own skin, you can do so by copying a current skin into a custom directory. The skins your site uses are located in \data\sites\1\skins (where 1 is the site number). Once you have your own skin directory, you can start changing the CSS files and the layout.master.
I suggest you check out the skinning documentation. http://www.mojoportal.com/skinning-overview.aspx.