Kenneth Fisher has a good case study on tuning with the help of Rob Farley:
Here we are looking at the difference between the estimated and actual number of rows for an element of the plan. To look at this information you can either mouse over the element or right click and open the properties tab. In this case you will see that the estimated number of rows (what the optimizer thought would happen) is fairly low (117) particularly compared to what actually happened (1494900). When you see a big difference like that in a query plan there is something wrong.
This is a really nice and detailed walkthrough in which Rob plays Socrates and Kenneth your favorite of the group (Thrasymachus anyone?).