In addition, research your price. Dont make the mistake of going in with a low price either. You will no longer be the customer but the tradesman. By that, the customers mind set is mostly that they pay too much for just having windows on their house washed, regardless what they are charged. Also a good rule, dont undercut just to get a job or to get your foot in so to speak. As long term you will regret it. You have your target income after tax in mind, so work backwards...what do you want to earn per hour/per day. Tax, travel costs, complications of windows/obstructions etc etc to be taken into account when determining your price and stick to it.
Hope this helps n again all the best