make sure that you did the following steps
1.copy the tools.jar of jdk and paste it in Tomcat/Common/lib and set the classpath for that.
2.Also set the class path for servlet-api.jar
3.And to run the servlet we should firstly follow certain format
For ex I am doind a student project in which iam adding a student
First in Student Folder(f)
/Student
/WeB-INF(f)
/Classes(f)
/AddStudent.java
/lib(If any jar files required)
/web.xml
/addstudent.html
After executing the java files place the entire Student Project in Tomcat/Webapps.

Hope you got the idea.