Choose an appropriate medium for the type of building you would like to draw. If you wish to make a very clean, architectural drawing with an emphasis on technical accuracy, choose a very precise medium like pen and ink. If you wish to make a drawing that has an organic feeling to it, use a messier medium like charcoal or pastels.
Choose a subject by looking through photographs of old buildings until you find the right image to replicate. Drawing from a photograph will allow you to include details that you otherwise might not have thought to include--the way the siding has peeled away from the building, the way that the shingles rest on the roof, the way paint peels in large chunks--and by extension will make your drawing more interesting to viewers. These details enhance your drawing, making it more realistic.
Old barns and buildings often convey a very specific message to viewers. These subjects often evoke feelings of nostalgia, whimsy, abandonment or neglect. As you draw your subject, ask yourself what feelings your subject conveys. Emphasize these qualities in your subject by adding or subtracting elements in the drawing. For example, if your drawing of an old barn is meant to convey an atmosphere of abandonment or uneasiness, subtract elements from the drawing so the barn's surroundings to barren. In addition, the way you handle your drawing tools should reinforce the atmosphere you intend to convey. For example, in the case of the abandoned barn, use a light hand as you draw, making your marks soft and eerie, perhaps even ghostly.
Learn to draw using one-point perspective before beginning an architectural subject like a building. In one-point perspective, you will establish what is known as a "vanishing point". The vanishing point is a spot in the distance where lines not parallel to the picture plane appear to be converging. Place the vanishing point--a dot that you can easily erase--somewhere on the horizon. Next, establish the side of the barn closest to you--the side parallel to the picture plane--by drawing a rectangle. Finally, draw the other visible side of the barn--the side that appears to be receding into the distance--by angling the top and bottom of that side of the barn toward the vanishing point. Show the rear of the barn by drawing a vertical line that connects the top and bottom of the receding side. Use a ruler.