As an introduction to similar triangles, present students with the Illustrative Math Problem: https://tasks.illustrativemathematics.org/content-standards/tasks/603

Students should work in partners and then present their thinking.  This opening activity can lead into a discussion of what information is needed to prove triangles are similar.  

After the opening discussion activity, students should work Which Triangles are similar and the Similar Triangles short cuts activity from the VDOE Mathematics Instructional Plan http://www.doe.virginia.gov/testing/sol/standards_docs/mathematics/2016/mip/geo/mip-G-7-similar-triangles.docx

These activities will help students develop an understanding of how to prove triangles are similar.  

A great exit ticket to this lesson could be the use of the quizizz game found here:  https://quizizz.com/admin/quiz/57e2d5617c807cb564e8c9ad  You can assign the quizizz as a homework activity or use live at the end of class.