You need to make 12 columns, one for each month, then put how much you expect to earn from each aspect of your business (domestic carpet cleans, domestic upholstery cleans, commercial cleans etc etc) If you put it that you will be busier in the Winter, you need to explain why you think this is. You should also breakdown all your expenses under these - fuel, insurance, chemicals, equipment, repairs etc. The difference between the two is your anticipated profit, if you're really honest with yourself, you'll get an idea how viable your business is.
You'll also probably find that you don't expect to earn back all of your initial investment in the first year, this is a common thing with a new start up so dont let it put you off (unless you're local to me, in which case - GIVE UP NOW!
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Hope this helps and makes sense...
Ash