Open the pet / mount window and there should be minion window on the right where you can assign your minions on missions.
You cannot summon minions, they are only used for minion missions.
You assign the minion on mission and if your minion is successful on the mission, you get some rewards out of it. It can fail too, but I cannot remember what happens if you fail.
It's similar system like Followers on World of Warcraft's Garrison.

Check the guides section on these forums, usually there is guide about this stuff.
[Guide] Minion & Servitors

There should be one icon for minion missions (I believe it's called "servant mission" in the latest english patch), where you can sent your minions to simple missions with a certain percentage of success (up to 3 minions per mission, up to 3 missions per day - You can refresh them with AP/fatigue). The rewards are exploration points - those points you also get for monsterbook quests.
Minions, unlike pets and mounts, cannot be summoned in any way.

Hope this helped you. There's also the minion & servant guide if you need.