From Crea Wiki
To place and destroy blocks you need to be in "building mode" (default "R"). Once in building mode you can equip blocks onto the hot bar and change your pickaxe. The items on your toolbar can be selected with the number keys 0-9 and once selected placed with the right mouse button. You can also hold the blocks on your mouse cursor by left clicking them in the inventory and placing them in the wold with left click. with your mouse and left click.
Background tiles can be placed by holding the shift key before placing. You must hold shift to place tiles in the background layer. All tiles can be placed in the background or foreground.
Similarly to placing background tiles, destroying background tiles is done by holding the shift key while using your pickaxe.
Objects such as chests and workbenches can be flipped by pressing the shift key before placing them. This is helpful for orientating your doors correctly.