My standard answer is whichever class you enjoy playing the most, as all classes can solo to 60. However, if you’re new to MMORPGs and you don’t know which class you enjoy playing that statement isn’t very helpful.
If you don’t know whether you would prefer playing a spell casting class or a melee class, I would suggest a melee class for your first character. They can take the most damage without dropping dead and that will give you the most leeway when learning game mechanics.
A warrior is a simple, strait forward class to level. You wear heavy armor and you beat on thing until the fall down. The down side is that warriors are more gear dependent than most classes. This means that you have to upgrade your armor and weapons more often to have the best chance of staying alive.
Rogues are also a good class. They only get to wear leather armor, but they kill quickly enough that unless you get jumped by multiple mobs you kill things quickly enough that leather is all you need. Rogues can stealth through areas and thus avoid mobs others have to fight through. They have multiple means of escaping a bad situation. And at level 20 they can do a quest for the ability to use poisons on their weapons. Rogues can get away with mediocre armor as long as their weapons are kept up to date.
Paladins are, more or less, fighting priests. They get to wear heavy armor like a warrior, but they lack a warrior’s attack skills and thus kill more slowly. They get to heal and raise the dead like a priest, but they don’t have the same range of spells, their heals cast slower and they don’t have as much manna to back up their spells. It’s a good class, but in my opinion it’s the hardest class in game to play well.
Hunters are also a very good solo class, if you enjoy having a pet at your side. Generally you send your pet to attack the target and keep it busy while you shoot it from afar. As the pet takes most of the damage there isn’t much down time. Hunters get a faster move speed at level 20 and at level 30 they get the ability to play dead and thus avoid many deadly situations.
As I said, every class can solo to level 60. So if another class sounds interesting go for it. You can have up to ten characters per server and fifty characters per account. So feel free to create multiple characters to see which you enjoy most.