When you click on a contact's email address, it will open up a new email window using your default email client. This setting is determined by your computer, and computers generally aren't set up to use Gmail as the default email client (because it's web-based). Not a problem. Here are two ways you can set up Gmail as your default email client:
Option #1: Install Gmail Notifier
Google has a program called Gmail Notifier which does a few things. It's primary function is to show you notifications when you receive new mail, but it will also make Gmail your default email client. You can install it for Windows or Mac.
Option #2: Install a Google Chrome Plugin
Our favorite web browser is Google Chrome. If you use Chrome, you can install an extension which will automatically convert all email links so that they open in Gmail. You can download Chrome here. Once you have Chrome installed, you can install either of these extensions: Mailto (recommended), or Send from Gmail.