You need to show two parts:  your projected income and your projected expenses.  They should at least come close to balancing out.  Have your income organised and broken down into how much you anticipate bringing in with ticket sales, donations, memberships, sponsorships and advertising, other sales such as gift shop, concessions, etc.  Expenses will be much the same way:  rent, utilities, insurance, production expenses and royalties, advertising, dues and subscriptions, postage, etc. 

The idea of a budget is not to force yourself to be completely and entirely accurate, but rather a road map.  You will probably forget things and that is really ok.  And maybe a year from now when you look back on your proposed budget for the year it will look out of wack, but that's ok, too.  You just need something to be your guide as you proceed through the year.

I have written almost all of our organizations budgets and that is what I do.  I used to really worry about it not being accurate, but others will much more experience assured me that no one was going to hold us to every dollar shown.  Hope this helps.