In other words … what do you want to be able to do?!
Clerics are a good choice for someone who wants to play a character who can fight and be able to cast spells. Clerics can do both fairly well, and they tend to be a good character that can slip back and forth between Main Character and Support Character without difficulty. A Cleric may be bashing in heads one second, and then mending wounds the next. It just depends on what is deemed most important at the time. Players who like slipping in and out of focus during sessions will want to play a Cleric. Just remember that you’re more than a walking first aid kit.
A Fighter is a skilled veteran of war, usually having a background as a soldier or mercenary. E. Gary Gygax called these guys “Fighting Men” but the term developed into “Fighter” as not to exclude women. A Fighter is a juggernaut in the course of battle, taking bold action and defending his weaker comrades. The Fighter tends to get a lot of action during adventures, but can easily be the one that goes down first. A player who likes a lot of action might want to play a Fighter, but be prepared to use your wits as well as your steel, or your Fighting Man will meet a quick end.
A Thief is probably the most misunderstood and stereotyped character in the game. Thieves in Fantasy have ranged from Bilbo Baggins to Conan. And remember, Robin Hood was a thief too. In basic terms, the Thief is a clever fellow who is adept at stealth, lockpicking, and removing traps. Although they do steal, it is assumed that they will not be stealing from members of their own group. Most Thief characters belong to a Guild, and owe loyalty to that organization. A player who likes to play the supporting role will fit right in as a thief character. Your character will not be dropping orcs by the dozen, but you will find and disable traps along the way which allow your comrades to avoid a horrible death. And frankly, who wouldn’t want to play the character who gets to lurk in the background until that perfect moment to POUNCE and defeat the bad guy at a crucial, defenseless moment?
Elves are good at fighting and casting spells. They are good at finding secret doors, and are immune to some of the ill effects caused by the Undead. A player who wants access to decent fighting ability as well as magic will want to play the Elf. Although Elves are tough, they generally play a support role. It really depends on how exactly a player chooses to use the abilities available to the Elf. The difference between playing a Cleric or an Elf is simply what kind of spells that the character will be able to cast.
A magic-user is a powerful student of magic. They are okay at combat, but not overly skilled in hand-to-hand combat. Magic-Users generally play a complete support role by staying in the back or the middle of the party, tossing a limited amount of powerful spells at the enemy. A player who chooses a Magic-User should understand that their character will be somewhat fragile, but will grow in power quickly over a short amount of time.
Halflings are kind of a mix of Thief and Fighter. They tend to use their smaller size to hide, and then strike when an opponent is least expecting it. Anyone familiar will the Lord of the Rings movies has ample examples of how Halflings can rise to the occasion to make a difference during a given adventure. A player who is able to think quickly and come up with imaginative ways to overcome monsters is best suited as a Halfling character.
The Dwarf is a lot like a Fighter, but also has abilities useful underground. They can read runes, find secret doors, find directions and depth while in a dungeon, and generally are tough. If you want to play a Fighter but want some special powers within dungeon complexes, choose the Dwarf character.
It is important to remember that the player and the character are two different persons. The more the two are kept apart, the better your games can be. The more the two are kept apart, the better your games can be. You are an actor, playing the role of a character in a story. Have fun with it!