Download and install Eclipse 3.3.2 (if you have an older version).
Start Eclipse (if you had an older version, set the workspace to the old workspace and all projects will be automatically imported).
In Eclipse, select Help > Software Updates > Find and Install, check the Search for new features to install box, click Next, choose the Callisto Disovery Site, press Finish, choose a Mirror site, select all features you want to install, this must include Webtools and all packages required for it. So click on Webtools and then press the Select Required button. Start the update.
Choose Window > Preferences then from the list Server > Installed Runtimes, click Add, choose Apache > Apache Tomcat 5.5, click Next, click Browse, choose the directory where you installed the Apache Tomcat application server (probably \Programme\Apache... \Tomcat 5.5), press Finish. You should now let Eclipse change to the Servers perspective where the entry for your tomcat server can be found.
Click File > New > Project... then select Web > Dynamic Web Project, enter name, in the field Target runtime the Apache Tomcat 5.5 should be visisble. Click Finish, for the licence agreement answer I Agree, now you should have a new web project in the workspace.
You can now add a new Servlet by right clicking on the name of the web-project, selectiong New > Servlet. A html page for the servlet can be created in the same way (New > HTML).
Write the Servlet and the html, then run it with Run As > Run on Server choose the Tomcat server, later you only have to start the tomcat server in the Server perspective.
For testing create a web-application, then a servlet named TestServlet, and a html page called index.html. Copy the code from the file TestServlet.java into your TestServlet and the code from index.txt into your index.html. Run the project with Run As > Run on Server. Your web-appication is now running on your application server. If you modify the html or the servlet the server will be updated automatically. If you restart Eclipse, you only have to start the application server in the Server java perspective.