See obtaining Thetis for installation instructions.
Once Thetis is successfully installed, you can start running example simulations that demonstrate basic functionality.
Setting up Thetis simulations makes use of Firedrake objects, such as meshes, functions, and expression. In order to be able to follow these demo simulations, it is essential to have a basic understanding of these objects. Please refer to the Firedrake manual for more information. Defining variational problems page is a good primer for understanding the Firedrake concepts.
The complete list of all the classes and methods is available at the thetis package page. The same information is indexed in alphabetical order. Another very effective mechanism is the site search engine.