So, now you are familiar with the basic dimensioning tools, it's time to start adding dimensions to our drawing. For this, I will be using the drawing shown here, where vertices are labelled with letters:

In the following example, we will learn how to add our first dimension to this sample drawing and we will start with the Linear dimension:

1. Click the arrow right next to the Linear dimension tool on the Annotation panel on the Home tab, as shown in the following screenshot: 

2. A menu with a list of dimension tools will show up, click Linear from this list of dimension tools.
3. Now click on point A and then on point B and then move your cursor away from line AB and click again to place the dimension.

You've added your very first dimension in the drawing and, as you might have noticed, AutoCAD will automatically find out the length of the line and will put that length on your dimension line:

You can add this dimension in a slightly different way as well, using the following example:

1. Select the Linear dimension tool once again and then, instead of clicking on any point, directly press Enter key.
2. Now click line AB and then click again some distance from the line to place your dimension.
3. You will notice that, in this case, AutoCAD automatically selected the endpoints of the line AB and will place the dimension accordingly.

The dimensions added here are associative, which means if you change the geometry, the dimension will update itself. So, in this example, if you increase the length of line AB, the dimension will automatically update to reflect the updated length of the line.