Cosmos, I'm not sure if you are asking how to create tabs or how to program them.
To create tabs, you can use CSS as described in this article.
To program the navigation, you can use the struts tabs tiles as described in this onjava article. This approach involves going to the server side and works in all browsers. An alternate approach is to use DHTML and switch tabs client side. This only works in more recent browsers and is more work (given that you are already using struts.)