FF IV is probably the best one to go with. The party is chosen for you, the upgrades and magic spells are pre-programmed for you. In FF V, you have to worry about which job classes you should have for each section of the game. In FF VI, you have a multitude of characters to choose from and build up. It's about twice as much as the characters in FF X. So if that was a bit much to deal with, then FF VI might be daunting. FF IV does it all for you, however. It's simplistic, and a good choice for a beginner.

(FF V is super awesome though!)