Forecasting your sales is the most difficult, most important part of any business plan. The bottom line is you just have to start somewhere.
First, how big is the overall market? Find out
How are you advertising your product? How many people will see your ad? Once you know this, you can estimate the percentage of people who will become purchasers. Then calculate the units sold.
What are your competitors' revenues and unit sales? Once you know this, you can figure your sales will be some percentage of this number.
How many salespeople will you hire, and how many units and dollars can each one sell per month? Once you have these numbers, you can calculate the monthly revenues you'll recognize.