Well, if Tomcat is configured following the standard configuration, i.e. you haven't altered where servlets go, then servlets (you really only need to put the .class files here but you can also put the .java files here if you want also) should be placed in TomcatHome\webapps\ROOT\WEB-INF\classes.
To access your servlet, start Tomcat, and assuming you're using the default port number 8080, type
http://localhost:8080/servlet/YourServletName into your browser.
This should work provided you've got Tomcat configured correctly - if not, there's a wonderful step-by-step guide to configuration
here Hope that helps,
Gillian