Android : how to create our own new activity in android using java code

I m new to android.I have create a new activity in eclipse automatically by following the procedure of creating new application using new file option in tabs.but i want to create a java file using java code alone that too i want to type it manually.i have gone through various searches and links like developer site, technopia but could get my answer clearly and i want to know how the newly created file gets executed. what shall i do in manifest file for its execution.please explain the answer. thank you.