edit: boo Marty!
First of all: each area of the game is designed for a full team of players, therefore you should always try to take the full team when you can, this will make the area easier for your team to complete.
The is no way to control individual henchmen positions, you have a henchmen flag but this will move all the henchmen in your party to the flagged location. This means you cannot send a Warrior henchmen into a fight without also sending the Monk and Elementalist henchmen in after him.
Which henchmen you take doesn't really matter a great deal at your stage of the game, as you play and learn more about what works and what doesn't you'll get a better idea of which henchmen to select.
Warrior Henchmen (and heroes to some extent) are not very good at positioning, they will switch targets and therefore easily lose their effectiveness.
Monk Henchmen are ok and will keep your red bars up as long as you control your aggro sensibly.
Elementalist henchmen often carry a lot of Area of Effect (AoE) spells so are good if you can bunch up the enemy.
Mesmer henchmen are useful for shutting down enemies particularly casters.
If you put the names of the henchmen into the wiki then you can see what skills they have and which skills will complement your build.