Mockup? Like designing? Pencil and paper.
If it's a matter of free time:
Once you get a strong grasp of HTML and CSS, you should study HTML5 and CSS3.
You should know every browser number a property is supported in by heart and know the exact prefixes needed (-webkit, -moz, etc).
You should be able to solve any problem posted in this forum.
After that Jade
Don't waste time with a photoshop mockup lol... there is much to learn.
The best designs are thought out in your head and are tweaked as you make them happen, it probably won't come together on photoshop.
Find certain aspects you like on other website and keep a general goal in mind.