if you're a F2P player you are limited to two characters.
Each class is mirrored in the other faction from a game mechanic point of view. Their skills will look different, but the rotation / prioritization will be the same.
The class stories are different for each class, but it will be the same for the two advanced classes within one class (i.e Jedi guardian and sentinel have the same story, but different from the sith juggernaut and marauder)
Jedi knight // Sith Warrior (tank /melee dps or pure melee dps)
Smuggler // Imperial Agent (heal /melee dps or ranged dps)
Jedi Consular // Sith inquisitor (heal /ranged dps or tank /melee dps)
Trooper // Bounty Hunter (heal /ranged dps or tank /shortranged dps)
Your starting planet may differ (All Jedis start on Tython, Sith on Koriban, Smuggler and Trooper on Ord Mantel, Bounty Hunter and Imperial Agent on Hutta) but you'll be reunited around level 10-11 on the fleet space station.
An option could be that one of you take the shuttle to the fleet as soon as possible and travel by shuttle to the other's starting planet. So you get to see each other's starting storyline.
If he is posting here he is subscribed :P