In Microsoft Excel, you need to enter your data in columns, and then click the Insert tab at the top of the page, and then find the Charts section. Choose either a line chart or a scatter chart, and click on it. This will insert a chart based on your data into the sheet you are working on. Double check to make sure all your data points are included. (If not, highlight all the data you want included in the relevant data column(s) and then repeat the instructions above.)
Once you see the chart, click on the body of the chart. Then click the tab at the top that says Layout. In the Analysis section of Layout there is an option that says Trendlines. Click on it, and at the bottom choose More Trendline Options. Pick either linear or exponential depending on which seems to fit your line better (use linear for a straight line, exponential for a curve), and add a number to the Forecast Forward option. If the extrapolation isn't long enough, go back and increase the number.