There are a lot of things that can cause issues with your audio setup.

The most important thing to keep in mind is that there must be a working microphone and speakers on both sides of the call. For example, if your friend can not hear you it could be a problem with your microphone or with their speakers or both.

There is a basic guide to diagnosing audio issues which is a good place to start. If you are still having difficulty, Skype Support has a guide on solving call quality problems.