Start with HTML 4 strict (there's no reason to use transitional for new pages). HTML 5 will only be adding new tags so you will not need to unlearn anything when HTML 5 is finally finished.
Also validate your HTML with the W3C validator at http://validator.w3.org/
so as to find and fix any errors (there are a few errors the validator can't detect but it does find most).
Helping others to solve their computer problem at http://www.felgall.com/