Pick up a rigidbody and have it remain still? (java)

I am very new to programming, and I have tried for hours trying to get this to work how I want (to no avail)

I need to be able to pickup a rigidbody when the mouse button is pressed, and have it remain in the bottom left corner of the screen, and drop it when the mouse button is pressed again.

In future, I will be rotating these objects to allow the player to build, but for now, I just want to be able to pick things up.

Any help will be greatly appreciated ^^

Thanks - Blaze