First, brainstorm what they already know about triangles and write their responses on a chart.
Show some examples. Perhaps print out some pictures of various triangles, or have some actual manipulatives, put them in a bag, and distribute a bag to small groups. They can look at and discuss, then see if they can add to the chart. Ask them where they see triangles in the real world.
Show this video. It's a great 2 1/2 minute you tube video clip on "Real World Triangles." http://www.youtube.com/watch?v=X6bQft45LXs