Revenue is defined as the value you gain in exchange for products or services. You can calculate your revenue margin by multiplying the sales of a certain period with the corresponding price. For the overall revenue, you just add up all the product/service revenues.
Banks and investors, who read your business plan, will thoroughly analyse both your revenue forecasting and strategy. If you are not able to sell your product to your customers, your startup will fail. Your backers know that in most cases revenue projections show wishful thinking or have been altered so the income covers the expenses. That is why it is in your own interest to base your calculations on the best estimate of the market, you can achieve.