New businesses have to make assumptions based on market research and realistic judgement.
Do you have customers waiting to buy from you? How many? Have you asked potential customers if they will buy from you? How much will they pay? If you have no idea where your customers will come from then you need to go back to your marketing strategy and identifying your market and potential customers.
If you have done market research then you could begin a sales forecast by listing the number of customers you think is realistic to achieve in your first year. List them as 'New customer 1', 'New customer 2' etc. Then try to assign a realistic sales figure against each of them. There must inevitably be some guesswork here, but at least you should reach a forecast that is broadly in the right area.
Being over optimistic with your sales forecasting will not help your business. New businesses should avoid the mistake of working out the level of sales they need for the business to be viable, then putting this figure in as the forecast.