Roughly put, for a new player E/Me is a good choice which teaches skill synergy, i.e., how certain combinations of skills are stronger than any of them on their own. Another good choice is E/Mo which teaches team synergy, i.e., how a team of players is stronger than any single member on their own.
Once you progress far enough through the game you will be given the option to change your secondary profession at will, and it will then be beneficial to try out all the secondaries to see how they can support your primary. Just keep in mind that your character is most effective when played in a way its primary profession was designed for. A tanking E/W with an axe is just that, ewww ...
