Altium - How do I place my GND vias?
You place a via and assign it to the ground net. That's it. 

If you are lazy, like me sometimes, I sometimes place the via directly on the net I want the via to be - this can be another trace, polygon, even a pad (and then move it later) - and Altium will automatically assign the via's net to whatever net you placed the via on. 

You should probably change your rules so that vias don't add thermal reliefs. You'd want direct connect (unless you plan on soldering things to it).

Create a rule similar to the image below, and that will make all your vias direct connect.