This question has come up a lot as of late. It's all about how you want to play really, you could make another character to do other classes on, but the game is designed so you can play everything on one character.

The other thing to consider is you'll probably get a bit of gear for other classes while you go through dungeons and do leve's.

Starting a new class is as easy as turning up to the guild for the class you want to start,getting the weapon you want from them and equipping it.

Hope any of this helps and good luck with your summoner