CodeAcademy is pretty good. This one talks about if…then: http://www.codecademy.com/courses/conditionals-in-javascript/1/1
And for document.write(), it basically just spits whatever you pass into it onto the DOM. So since a span is just: <span>something</span>, you just do document.write("<span>something</span>").
Edit: haha yea what @WolfShade said