There are a lot of ways to do that. Simplest would be straight line forecasting:
next month sales will be the same as this month sales.
next six months sales will be the same as the last six month sales.
Better is to have more data (i.e., do sales usually go up in winter, summer, spring, fall?
As well as better to have more input from marketing and production (i.e., will there be major marketing campaigns designed to increase sales? Is there a new product release?)
Also you can use inputs from overall macro economic trends (is my industry overall in a growth trend or not? What kind of major events might impact growth in my market?)
You can access regression functions in Excel. Often those formulas used without any other inputs or analysis isn't much better than just tracking moving averages.