You don't have to use Ant to build your project, but if you do and you'd like to save yourself some time, I recommend you use seam-gen to create the project structure (and it will also generate a build.xml for you).
There's a step by step guide on how to do this in the reference docs, which you can find here.
In Seam 2, your options are Ant (in which I highly recommend seam-gen) or Maven. seam-gen will generate a very decent build.xml with lots of useful targets (just see the booking example's build.xml). Seam 3 will be based on Maven. When using Maven 2 with Seam 2, you're basically on your own AFAIK...
Thank you guys. That was very helpful. I tried and compared SeamGen and JBossTools. First SeamGen was better for me as it generates build.xml for me. But then I found out that I can write or use build.xml from other projects and change it for the project generated by JBoss Tools. So it worked fine with JBoss Tools generated project. Also, I can deploy the project strictly to JBossServer/server/default/deploy just like deploying by ant and seamgen.