That is entirely up to you. If you want to go straight to 80 on a single class, no problem. If you want to take it slow and level everything, that's fine too. Unless you have some specific goal in mind with regards to leveling there's no right or wrong way to go. Just do whatever seems fun. It's not like there's a race to 80 you can lose if you level too slowly.

As for easy and fun classes, that's mostly a matter of personal preference. All classes are easy to learn at lower levels and ease you into the more complicated aspects as you level up.