You can create a Servlet in the IDE by selecting Servlet from "File->New" or right clicking in the Project Explroer -- it is one of the options. If you take this route, you will get a class with code filled in already implementing all of the Methods, you can just add your own logic.
Alternately, if you have an existing servlet or want to hand-code your own, you can use 'Mark As Servlet' which I beleive is under 'Tools' ( don't have my IDE handy at the moment).
If you have a Servlet and go to 'Execute', NetBeans will launch an internal Tomcat process and excecute your servlet to it, as well as bring up a browser window and point you to the correct port on localhost so that you can view it. You can specify which browser under "Options".
disclaimer: I still use NetBeans 3.4.x .. haven't upgraded to 3.5 yet.