If you mean inside the barn, you'll need a tripod, or someting solid to rest the camera on. Most barn interiors are not well lighted, and without a flash your shutter speed will be too slow to allow hand-holding the camera without blurring the image due to camera shake.

Use only the center focus point; there will be instructons on how to do this in the manual. You will probably have to hold the shutter button 1/2 way down and recompose.
Boost the iso up to 800 or higher; this will give you the shutter speed you need for hand held shots.
The aperture you need to get the right depth of field will depend upon how close you are to the animal and wether you are going for a full body or a head shot. Somewhere around F4-5.6 should do it.