If you don't know, LED's have a positive side and a negative side, and will only light up when wired correctly. Most LED's have one longer lead to indicate the polarity, but not all. Here's a simple solution to find the correct side BEFORE installing it into your project.
Step 1: Equiptment List
2: Wire strippers/cutters. C'mon, you don't have a pair these in your back pocket already?