A good database design helps prevent you from duplicating data. It also helps ensure your data is complete, and most importantly, that it's accurate.
To reach those goals, start by listing the data you want to capture. You can start with your existing data — in this case, your spreadsheet. Or, if you use paper ledgers or forms, gather examples of those. And don't hesitate to ask your coworkers what they need.
Another way to identify the information you need to store is to create a flowchart of the tasks associated with your data. For example, who will enter the data, and how? What kinds of forms will they need? And while you're at it, think about the reports or mailings you want to produce from the database. For example, do you want to know when desks and chairs need to be replaced? Who needs that information? Looking at the data you need to enter and consume can help you decide which data to store.