Creating a basic GWT app
In creating a basic GWT app you will want to go to the GWT website and go to this page http://www.gwtproject.org/doc/latest/tutorial/gettingstarted.html. From here you will want to follow the first link in the what you will learn section create a GWT section. After following this you will have the basics of what you need to create a GWT app. From this point you can continue with the guide and create the stockwatcher app to learn how it works or if you want you can just take the basics of what was given to you and create your own application.
If you decide to write your own application there are a few things that you will need to know. You still have the basics of a usual java web app with the JSP pages, servlets and web.xml. From this point you can design your pages as you with and write your backend code to go with it.
There are quite a few resources that you can find online that can help you get through the process of learning how to actually use GWT and navigate the menu which can be fairly difficult. The ones that are on the GWT website are not bad but there are better ones in my opinion, such as the tutorial point guide. Other than that if you are a more visual learner there are quite a few youtube tutorials that you can follow as well, which one depends on you and who you wish to listen too. The last thing that I would mention is that when begining to program with GWT it can be quite frustrating so you will want to make sure to take breaks and not get stuck on one issue for too long, it will take some time to learn but once you have it is a very good tool to have knowledge on.