You made your plans. Now how well are you going to execute?
Forecast is a near-future prediction on your financial results. You know how good your sales are. Will they change in the following months?
Forecasts are basically top-down predictions based on statistics and past performance. You take a guess on how your sales are going to do based on your first two months, seasonality in your previous years and sometimes already indicated sales.
Follow that with expenses. You have two main categories, those expenses, that follow closely sales. Those are called Costs of Goods Sold, COGS. And general and administrative expenses are usually fixed and broadly independent on your sales and scale of production.
Check your COGS, assume how much of the sales needs to go towards them and how much is left for the firm. You will use some very basic mathematics like fractions and ratios, especially ratio of expenses to revenue.
G&A is going to be more tricky. But assume your personnel expenses to be constant, unless you expect find new hires. And unless you have clear indication otherwise, assume everyone will try to spend their budgets. It pays to be pessimistic.
In the end, forecast for the year is just a mixture of historical data from first months and quantitative predictions on the rest of the year, that are slightly more precise than the simple plan. Be careful to not believe all good news too much and employ your own precautionary principle.