Actually, Dreamweaver 8 has some tools that will help with this. I'm not familiar with versions of DW prior to 8, so I don't know if older versions have the same tools, but in my opinion, you need to do the following:

1) Convert the page type: From the file menu, choose 'convert', then set the type to XHTML.

2) Check your markup: Again, from the file menu, choose 'check page', then 'validate markup'.

Now of course, when you validate markup, any problems will show up in the code window, but Dreamweaver will give you warnings of what potential troublespots are. You'll still need some code knowledge to fix some of the problems. I agree with everyone else on the usefulness of w3schools for this, but at least these steps will give you a head start.