Game is similar to WoW, but on positive side it has no monthly fee.
On negative side, as Auros said, it can end up being more expensive then subscription game if you don't put good time to it, or don't have patience this game requires.
As for class, there are couple of combos that atm are easier then others to start with,but your endgame will strongly depend on classes you select for main character.
Some good ones to try R/S, M/P, Wd/D, Wd/S, W/K, P/K. Start from what you like to do, if is DPS, then R/S, for solo then anything with healer, great damage from far mage or warden, tank, then Wd/W or K/W... etc. It is free and you can tray as many combos as you like. (you are allowed to open 5 accounts, and each account can have up to 8 characters per server)
What server are you planning to join? PvE or PvP?