Filmmaking is a very expensive undertaking. The size of the project is expediential is aligned with the budget. This goes without saying, but in all cases the budget is very important, no matter the size of your project. The first thing you'll need is a screenplay. If you already have a screenplay, knowledge is the next step in creating a budget, knowledge in what it cost for everything you'll need, actors, cameras, film/tape/hard drives, food, locations, crew etc. If you do have a screenplay, then the first cost is acquiring one. That can be the first item that goes into your budget. There are basically two budgets in any project, a preproduction and a production budget. The preproduction will be for investors and studio heads, when your looking for funds to produce your film. A guess if you will of what it might cast. The production budget can never really be a final budget. Most production budgets are fluid, contracts with crew and cast may change and changes in the screenplay all effect your budget. When production starts the budget is somewhat finalizes. And sticking to the budget can be huge in your filmmaking career.

There are many computer programs for budgeting for films.
Movie Magic
Production Pro
Gorilla

Section A - Above the line costs (development, writer, producer, director, stars)
Sections B & C - Production and Post-Production costs
Section D - General expenses and indirect costs including corporate overhead, medicals, legals, insurance, financing