So, as per the rulebook, the players are free to select their leader. Keep in mind that while the player who is the leader doesn't have any authority over the actions of other players, if the team becomes stuck in an intractable disagreement I will ask that player to make a decision to keep the game from becoming stuck. In all honesty don't anticipate that happening, as usually I find players are quite able to to hash out such things without much trouble.
Mechanically, remember that beyond serving as a "tie-break", the leader can also repair squad cohesion and can put the kill-team in squad mode rapidly, so it is advisable to make a character with a high Fellowship and/or Command skill the leader.
Once a leader has been selected, the team will have to choose an oath for the mission as appropriate. Discuss the selection here.