Snap to Geometry will automatically be turned on to help you with placing the object in the scene.
You can also turn on Grid Snapping, Angle Snapping and Rotate to Surface Normal to refine your placement.
Click here for more information on snapping.
Use the Delete button on your keyboard to delete selected objects.
If you want to undo the deletion, you can use the Undo button or use the shortcut Ctrl+Z.
Learn how to create objects in the level and how to edit them.