Creating an IT Budget– from SCRATCH!

I could really use some help. I’m new to a small company(about 25 employees), and I’ve been hired to create an IT department. I can definitely handle all of the work, but I’ve never been at the management level before so I don’t know how to create a budget. My biggest issue is that there are no previous budgets for me to use as a model. Please can you give me some advice on creating a budget!