Hi lulu, and welcome!
you start out with either a warrior or mage and then at various levels you can change to a different class if you want. cleric is at L5, thief at 10, ranger at 15, etc.
As to which class is good for beginners: really any of the classes up to 20 are good classes and easy to play and play well. If you have some idea of what class you play best as, or that appeals to you most; do that. Note: after the first 5 or so classes, the new unlocked classes have requirements. For example Bladedancer requires that you be L50 character and L45 Bard.
This game is very fun and even more fun when played with friends/guild mates. I suggest you find a guild ASAP. A lot of guilds accept new players and will help you with leveling, tips, keeping you from getting bored, someone to chat while doing grinding. The leveling really isn't bad at all until around L40 or so. That is when you'll have to start doing daily (one time) quests to level, in addition to the storyline quests. Again having a guild is a good thing because you also have guild quests that have good exp/gold rewards (and they help the guild gain fame).
If you have any further questions my in-game name is Ogii and I can be found on either server. You can talk to me by: /w <your message here> in the chat box or press o (friend list) click add, type Ogii, then right click my name, choose whisper. to reply to a whisper you can either ctrl-r or right click the name in the chat box and choose whisper.
Hope this helps!