The easiest way I think is to look at an example build file with the Ant docs to hand. Look an an open source project (just grab something simple from Sourceforge) - most will use Ant and will include the build file. Small build files are relatively readable, even without much Ant knowledge. (Watch out for big projects like JBoss - projects like these tend to have expanded way beyond the limitations of Ant and use other products inc conjunction with it, or have defined their own custom tasks)
The most important thing to remember with Ant is to understand what you are trying to achieve with a build file. If you have a clear understanding of the steps in your build process, then translating these to an Ant build file is relatively trivial. If you don't have a clear understanding of the steps in your build process, sort this out before going anywhere near Ant.
There is also an Ant debugger available as an Eclipse plugin. This can be very useful to track what the more complex build files do.
ANT does all these things by itself. All you need to do is script it once to tell what needs to be done. Thereafter ... till eternity ANT will automatically do all the tasks for you... till you tell it not to do . ANT, just does not compile files, but also does a lot more things like running external utilities, building jars and ears, firing SQL queries, deploying and starting applications, sending mails and a lot more. So an entire cycle of building to testing to reporting can be performed with ANT once the code has been checked in with just one command that invokes the ANT script.