You need a Tomcat eclipse plugin such as sysdeo. The sysdeo plugin (http://www.eclipsetotale.com/tomcatPlugin.html) will allow you to launch Tomcat from within Eclipse and use your web browser while setting breakpoints in your webapp source code.
Actually, you can bring up Tomcat in remote debugging mode and connect Eclipse to the stand-alone Tomcat, but usually sysdeo is more convenient.
An IDE is no substitute for an Intelligent Developer.
You don't really need a third-party plug-in. If you've downloaded and installed the Java EE Eclipse bundle, then you don't need to install anything in Eclipse. Otherwise, if you don't have the Web Tools and Java EE tools in Eclipse, you need to install them first via the Update Manager, which you can find under Help / Software Updates.
Then look at Window / Preferences / Server / Runtime Environments. Click the Add button to add your Tomcat installation there.
Also see the help included with Eclipse, it explains it in detail.