You will have to update the classpath of the project that uses the javax.servlet.*
classes in Eclipse with the necessary jars containing the javax.servlet
and related packages.
So select the project (left pane), right click → Build Path → Configure Build Path. Go to the "Libraries" tab, select "Add JARs" and navigate to the JAR(s) containing the packages you want. These will probably be in the installation of the EE SDK (you will have to search a bit).
BUT...
You will probably have better luck installing Eclipse for JEE developers (currently here). It will open by default in a new Perspective (the icons on the top-right of the toolbar): JEE instead of Java.
Then create a server of the type you are using (for plain web projects, Tomcat would suffice): Window → Preferences → Server → Runtime Environemnts → Add. You will have to download the runtime for your server, if not provided (Tomcat is provided) and select the installation directory.
Then create a new server with this runtime: Window → Show View → Other → Servers. In the opened "Servers" view, right click and select New → Server. Give it a name and the Runtime environment you created above.
Now create a new Web Project and assign it to the newly created server. From the servers view, you will be able to start and debug the application in the server, redeploy it etc.
These are quick instructions... If you are completely new to this kind of development, maybe you should seek additional help for topics that trouble you. ALSO you may want to experiment with a simple project (a servlet, a filter, a JSP etc) first to get acquianted with the tools.