Pega Academy is a place where you can find all the courses that help you learn Pega. At the top of your PDN page, you will find a tab called Academy. Hover on it to see the options, I would suggest you click on Course Catalog and choose the course that would be most helpful.
Here's the link to Pega Academy, where you could get more information on these courses! Pega Academy
When rule: if you want to show any section based on user then you can check his access group using when condition. It returns boolean value.
Decision table: there are list of letters in your application, which needs to send based on the work party. you should maintain the list of letters in one column and return the work party in that decision table.If you have situation like "If else if" condition then prefer decision table.
Decision tree: If you have scenarios like Nested if condition then prefer decision tree.
Map Value : If you have scenarios like matrix form to return a value, then prefer map value.