How to control the initial pose of robot

I have a robot in gazebo, every time when I run it, the robot always fall down from the sky. Is there a way to control the initial pose of the robot? The other problem is my robot is 24644 kg, really heavy. when I load the robot in gazebo, part of the wheel is under the ground. I already set the collision in my xacro file. Does any body have an idea? Thank you