This video explains the main things you need to know about similar triangles and polygons: what they are and how to spot them. It also covers types of polygons that are ALWAYS similar to each other like squares, equilateral triangles, special triangles, etc.
Sides of Similar Polygons
This video covers a bunch of examples of the most common type of question you'll see about similar triangles and polygons: finding the lengths of missing sides (solving for x) between figures you are told are similar.
Scale Factors Between Similar Triangles & Polygons
A scale factor just tells you how much bigger one shape is that another that it's similar to. If something is twice as big, the scale factor is 2. Three times the size, the factor is 3. This video explains how to find the scale factor, and then how to use it once you have it.
If you do not have an account, you should get one, because it is awesome! You can save a playlist for each test or each chapter, and save your "greatest hits" into a "watch right before the final" list (not that we recommend cramming, but when in Rome...)