In your Eclipse project, got to Project | Properties, select
Java Build Path then click the Libraries tab. Then click Add JARs and browse to the location for the file within the Tomcat directory.
By the way, since you are new to Eclipse I highly recommend reading the help text (Help | Help Contents), especially the Workbench User Guide, Java Development User Guide and Web Tools Platform User Guide. Those topics should tell you almost everything you need to be productive with Eclipse.