I would start by making graphs and similar things. CloudDeploy and FormFunction are useful once you've learned the basics of the language.

I would take a look at the virtual book for an introduction:

http://reference.wolfram.com/language/tutorial/VirtualBookOverview.html

There are more resources in books and videos you might find helpful:

https://www.wolfram.com/broadcast/screencasts/handsonstart/

Restricted depends heavily on the type (its first argument). The documentation for Restricted doesn't cover its full behavior, just a very large set of common ways that it is used.

Look at the documentation for things like "Expression" and "MathExpression" under "Restriction Specifications". These documentation pages cover how Restricted is used with them.

http://reference.wolfram.com/language/ref/interpreter/Expression.html

The documentation pages for Interpreter types also have examples of how Restricted is used with them.