There are two major sets of factors that will influence the design and construction of your steel building. One is practical: the actual use of the building. This requires work on your part to think through exactly how you'll use the building. For example, if you're building a warehouse, you may need roll-up doors high enough for your forklifts to drive through. Steel buildings always require foundations, which in most cases are flat concrete slabs.
The second is legal. Every state has different building codes that will apply to your project. These will include things like snow load and wind load: how much of either your building must be able to stand up to. If you're building a church or emergency vehicle garage, extra "importance factors" may apply that require push the code requirements even higher. Other legal requirements include local zoning laws, drainage requirements, and more.
Some aspects of your design involve both practical and legal considerations: for a retail facility, appropriate parking and handicapped access fall under both categories.
While steel building suppliers can often help you research these codes - most won't ship you a building that doesn't meet codes in your area - the ultimate responsibility for meeting them is yours. The bottom line on planning your building is that if you're not familiar with major construction projects, you'll need to rely on experts to assist your planning.