Each class has it's own style, and each is good in their own way. Choosing your primary class is all about what you want to do in the game. Before I get into that I'll address Factions. You absolutely do not need Factions (the 'expansion', which is actually a stand-alone game) to play Prophecies (the original game) well, and enjoy it even in PvP. The games are balanced in a way that you can still be competitive in PvP even if you only have one of the two games. Of course having both gives you more options, but nothing game-breaking.
Here are the Classes you have to choose from, and a bit about each:
Rangers: They have moderate armor, so they're not "soft" targets like the casters, and are more self-sufficient. This is good for playing through the storyline of the game. Rangers have decent damage output, and it's ranged so they aren't close enough to get hurt as much as a warrior would be. Rangers can do many things well, such as trap, interupt, and can also have a pet, which can help almost as much as another player in some cases. It's what I chose as my first character, and in my opinion it's great to learn the game dynamics on.
Elementalists: "ele"s are great at first glance due to the massive damage output. Though they're good at mindless spamming of damage, they generally don't really offer much in the way of strategy, and I wouldn't recommend them for your first character. This isn't always true of course, as an experienced player can use eles with great strategies, but as a new player I'd recommend something else.
Monks (my favorite): Fairly self-explainatory, they heal and protect their allies. As such, they're everyones favorite friend, since most players like doing damage and just expect others to keep them alive. This ensures you a spot on any group, as all groups (think) they need at least 1-2 monks to be successful. They're usually right.
Mesmers: Mesmers are very fun to play, because their whole existance revolves around stopping others from doing whatever it is they wanted to do. Most of their damage is passive, and requires the enemy to trigger it in some way. They can be very effective in this regard, dropping casters and melee-based opponents with humiliating efficiency. Mesmers are a harder class to understand at first, due to their limitless complexity, but are very fun because of it. A problem with mesmers is that most PvE players don't understand why they're so good, and it may be difficult to find parties when doing missions/quests.
Necromancers: necros are similar to mesmers, but... different. They have a very unique set of skills, which is also very complex, like the mesmer. Necros are extremely powerful, much moreso than the elementalist (when applied correctly) but also slightly underrated. Lately necros have been given more respect however, and you shouldn't have a problem getting into a group as one. The Blood line is also great for self-preservation, which is good for the early stages of the game.
Warrior: Everyone's first character

, it's just as you'd imagine, you hit things with pointy sticks. Theres not a lot of thought to playing a warrior for the most part (in PvE,) so it may be a good thing to warm up on while you learn the basics. The most useful thing about warriors is their high armor level, which makes them less vunerable to attacks. In PvE they're often used as "tanks" that sit and absorb damage, while the rest of the team kills.