IT budgets can be difficult to plan, because in addition to hardware and software, they should also include current IT maintenance, longer-term planning, and business development. Calculating your previous year's IT costs will give you a good range to start, provided you aren't planning a major change
If you don't know what new technology you'll need, begin with a request for proposal that lays out your technology needs, project requirements, and desired deliverables. Continue by shopping your RFP around to various IT professionals, who will give you a budget and a timeline for the project. If this sounds like a lot of work, take heart; with practice the budgeting process will become both easier and more accurate.
For more on budgeting, be sure to read How to Plan Your IT Budget.