Start with the basics. Buy some textbooks, get into the basic sentences, learn how the sentences are structured ands how the grammar works. Practice is the key. Using lots of different media is also helpful. Books, tv, films, music, whatever. You gotta put in lots of hours of being exposed to the language. Better yet, get to know some native speakers and practice with them.

Being skilled in a language in a certain language group (such as Latin or Slavic languages) definitely helps. You've got a headstart leaning Spanish or Portuguese because you already have some familiarity with french. The grammar is very similar, as well as a large part of the vocabularies.