Budgeting is really just common sense and being organised. I'm an accountant, so you'd expect me to say that, but really it is.
1. List out what furniture you want to have.
2. Price that list. Where can you get the best buys from? Now you've got a more detailed list - I want THIS table and THESE chairs and THIS bed, and they will cost THIS much.
3. Total it up - there's your total budget.
4. When you've recovered from looking at that figure, what do you need immediately and what can you leave until later? Split the list out into "now" and "later", maybe even more columns for how much later.
Spreadsheets were made for budgeting - if you have a spreadsheet program on your computer, use it. It'll do all the adding up for you. List all the items down the side, and what month (or other time period) you plan to buy each one across the top. Put the price of everything in the appropriate place in this table, and totalling along the bottom gives you what you're planning to spend each month or other time period.
Believe it or not, that's exactly what companies do to prepare a cash flow budget! They do an estimate for each month of what money they expect to come in, what they expect to have to pay out, and then they can work out if they're going to need to borrow some money to tide things over. It works just as well for anyone.