All the previous posts have offered solid advice. Approaching the model development while giving significant consideration to components may work well for a relatively simple structure such as a barn, but I suggest that it may be more appropriate to address general building concept issues first when developing a building model and then work out specific (component) ramifications secondary to the more definitive plan and vertical building features. This can be valid whether using SketchUp or any other assistive design software. You should give some serious thought to placement of partitions, including any desired openings (doors, windows, pass throughs, etc.) Decisions will be required relative to floor to floor heights, ceiling height above floor, roof configuration and pitch, plenum dimension (depth from floor to ceiling below the floor including space for ductwork possibly); merely to illustrate a few items for reflection during the process. Repetitive structural elements like beams, girts, trusses, etc. can be developed as components and inserted where space is provided. The same is true for doors and windows, of course.
Starting with drawing a rectangle on the ground seems to be a good logical point of initiation. I find it convenient to approach design in a somewhat systemic manner because it usually results in a more efficient use of time for me.