Cover

Cover in RimWorld can be found in four ways: Rubble, Sandbags, Walls and the Saguaro cactus.

Rubble

Rubble is littered all over the map and will be used by attackers to gain an advantage over you. You can also use rubble for protection, but the random rubble placement is unlikely to favour you. so it's advisable to use it as a last resort

Rubble in combination with a dumping area can easily give a turret some extra protection, for free! First designate a dumping area where you want your turret to be, then mark 16 rocks to be hauled to the area, when it is filed sell the dumping area and mark the middle 4 rocks to be hauled again but to another dumping area (designate a new area somewhere if you must), then build a turret inside the resulting square of rocks.

SandBags

Sandbags are the easiest and cheapest method of protection. U shapes are good for one character to hide in. Zigzag patterns will allow multiple characters to cover close together, which is handy if you want to focus fire on a few attackers.

Sandbags enclosing a turret make a great attack point But keep the turret repaired, as it will explode when destroyed and can damaged nearby characters.

Walls

Walls can be used as cover as well. Characters will lean around them to shoot. Three wall blocks placed diagonally make a great sniper nest. Natural walls work in the same way.

Saguaro cactus

It's a large tree like cactus with 2 branches, see this page for a picture.