You need to do quests in order to obtain the secondary professions. Please read the guides in the New Students Center as well as *all* of the FAQs that are stickied.
As for the original poster's question, there is no best class combinations in GW. It all depends on what you want to do with the character; do you want disruption? Caster-hate? Nuker? Healer? Tanker?
Please take a look at the strategy forums (The Campfire) for some examples and discussions on specific builds.