Unfortunately it would take way too long or take up way too much space to describe everything from beginning to end. Especially when each class story takes place in different settings and don't necessarily link with each other all too well. However, once you reach the expansions the story remains the same for all characters. If you don't feel like playing through too much, I suggest you start with the Zakuul expansions. However, you can always blast through the class stories by purchasing a Master's datacron from the cartel market. This will automatically boost your character to level 70 and would allow you to get through the class story without doing any of the side quests or flashpoints. I do highly recommend playing through at least one of the class stories, as they present a lot of structure to the expansions based off your choices.