I hope there are a lot of things about your business in your mind and may be you have written down a few things. You can proceed from there.
Download a business plan template from here.
write your company's or business' name, address, phone, fax number etc. on the first page. Read it aloud. This small act itself will create magic! You will feel encouraged to write further.
Go on filling information on the other pages. If you get stuck somewhere, leave it and go to next. Do more homework and get back.
Write real answers. Do not fool yourself.
Go out and collect as much information about your competitors as you can (don't say you don't have competition!) and write it there.
Also try and define your customer in as much detail as possible.
Some people find it easier to do this work on paper rather than on computer (I, for example!). If you are one of them, it is alright to first do it on paper and then type it out. A good number of improvements do take place in the process.
About financials: overestimate expenses and underestimate income.
About time lines: Remember, everything takes twice as much time, if at all it gets done.
After every major chunk of time spent on the business plan read it from the beginning. See if it makes sense.
Simultenously, start working on your marketing plan (download a sample as close to your business as possible from mplans.com. It is free.) this simultaneous working will help improving the quality of all three.
Just start. Start anywhere, but start. You will find your way.
My best wishes to you.
-- R C Kathale