How do i use signs or make an gui for people to select classes? And when they select iT get the items immediatly?

See the Star Wars class chooser for an example of how you'd set this up:

https://github.com/elBukkit/MagicPlugin/blob/master/Magic/src/main/resources/examples/stars/spells/starshop.yml

Magic has no support for signs at all, but I'm sure you can find some plugin that runs commands from signs. Or just use NPCs:

https://github.com/elBukkit/MagicPlugin/wiki/NPC