Question about writing a business plan...?

I'm wanting to start a photography studio, and need to write a business plan to see if my "wishes" are going to be realistic as far as finances are concerned. I can get a basic handle on what my start-up and monthly expenses will be, but I have NO IDEA what kind of cash flow would be coming into the business from customers. I do not know how many customers I will have a month, and for that matter, how much those customers will purchase in prints. It scares me to go into business not knowing if I will make enough from customers to pay for the business! How can I get an idea of how much money the business will make each month?

You might like to have a look at the business startup books with sample pages at:

http://www.browseinside.biz

Kay

Simply be reasonable and be able to back-up your figures with facts/reasons as to why you think it will be that way.
That doesn't mean it has to be exactly as you state, but you should be able to at least roughly estimate your sales figures each month and tell someone why you think it will be that way.

How many weddings/portrait sessions/kids/seniors/families/commercia... jobs etc. etc. and what will your average gross and net be per job/catagory?

If you honestly have no idea what to expect, then honestly, don't do it. You'll need more experience.