First make sure you have Eclipse for Java EE developers.
Next goto Java EE perspective, open Servers view, click to add a new server, choose your version of Apache Tomcat, select Tomcat installation directory and click Finish. This integrated your Tomcat instance with Eclipse and copied Tomcat configuration files into project Servers.
To start server simply press big green button. If you goto http://localhost:8080, you will see 404 message. That's OK, because right now we don't have any application deployed. This message is from Tomcat, so Tomcat runs successfully.
You can change lots of configuration if you right click on your server and press Open. You can change virtual machine parameters if you click on "Open launch configuration". There you can add Xmx and XX:MaxPermSize parameters.