I've always just selected Export from Eclipse. It builds the war file and includes all necessary files. Providing you created the project as a web project that's all you'll need to do. Eclipse makes it very simple to do.
You can then execute one click in Eclipse to run that Ant target. Here are examples of each of the steps:
Preconditions
We'll assume that you have your code organized like:
${basedir}/src: Java files, properties, XML config files
${basedir}/web: Your JSP files
${basedir}/web/lib: Any JARs required at runtime
${basedir}/web/META-INF: Your manifest
${basedir}/web/WEB-INF: Your web.xml files
Set up
Define a setup task that creates the distribution directory and copies any artifacts that need to be WARred directly:
<target name="setup">
<mkdir dir="dist" />
<echo>Copying web into dist</echo>
<copydir dest="dist/web" src="web" />
<copydir dest="dist/web/WEB-INF/lib" src="${basedir}/../web/WEB-INF/lib" />
</target>
Compile
Build your Java files into classes and copy over any non-Java artifacts that reside under src but need to be available at runtime (e.g. properties, XML files, etc.):
Once all this is set up, simply launching the default target from Eclipse will compile, WAR, and deploy your solution.
The advantage of this approach is that it will work outside Eclipse as well as within Eclipse and can be used to easily share your deployment strategy (e.g. via source control) with other developers who are also working on your project.
If you create a new web project in Eclipse (I am referring to the Java EE version), the structure is created for you and you can also tell it where your Appserver is installed and it will deploy and start the application for you.
Using the "Export->WAR file" option will let you save the war file.
Follow the prompted steps - choosing the simple java web project (18) and a suitable name.
When it is finished run: mvn eclipse:eclipse
Start Eclipse. Choose File -> Import -> Existing project. Select the directory where you ran the mvn goals.
That's it you should now have a very good start to a war project in eclipse
You can create the war itself by running mvn package or deploy it by setting up a server in eclipse and simply adding adding the project to the server.
As some others have said the downside of using maven is that you have to use the maven conventions. But I think if you are just starting out, learning the conventions is a good idea before you start making your own. There's nothing to stop you changing/refactoring to your own preferred method at a later point.
Another option would be to build it automatically using Eclipse. Of course if you have continuous integration environment Ant or Maven is recommended. The export alternative is not very convenient because you have to configure every time the export properties.
STEPS:
Enable "Project Archives" support; this might depend on your project (I used it on Java EE/Web project). Right-click project root directory; Configure -> Add Project Archives Support.
Go and create a new archive in the "Project Archives" top dir. You have only jar option, but name you archive *.war.
Configure Fileset-s, i.e what files to be included. Typical is to configure two filesets similar how the Web Deployment Assembly (project property) is configured.
copy /WebContent to /
copy /build/classes to WEB-INF/classes (create this fileset after you define the WEB-INF/classes directory in the archive)
You might need to tweek the fileset exclude property depending where you placed some of the config files or you might need more filesets, but the idea is that once you configured this you don't need to change it.
Build the archive manually or publish directly to server; but is also automatically built for you by Eclipse