After I create a settler I want to move it to my army and I don't know how to do this.

MythicAlex wrote:

Select the units you want to move out of your garrison, and then click the "New Army" button at the bottom.

You can merge armies by moving one army into another if there's enough space to fit every unit (this can be increased with research later), alternatively you can manually move units between armies by selecting units and then pressing the "Transfer" button when directly next to another army. This can also be used to create several smaller armies out of a single army.

It says there are no available postions to create a new army

You can't move units from a city garrison directly into an existing army, so if your city tiles are currently occupied by other armies there might not be any tiles left to put your settler until someone moves. If you're still having trouble you could upload a save and I can check it out myself, save locations are described here, and you can upload them using the button shown in this picture.