Press P to toggle editor mode and play mode
Click on a icon at the top of the screen and click again anywhere to place that object where your mouse is
Click on an object to select it.
Press R to deselect all objects
You can also click on an object again after selecting it to deselect it.
While an object is selected, use the w a s d keys to change the size of the object. (Unless its a spike or a Bomber)
Click and drag objects to move them around (Note that this will also select that object)