How to make a complete parser?

Question

Parsing

Good afternoon, help! I am new to c#. I seem to understand that there are variable cycles, methods, constructors, classes, but I don’t understand what to write so that the program starts acting with the object. Roughly:
- you need to parse phones and prices according to the catalog - cycles, requests, working with files, it’s clear where take a specific command, a request for the program to start searching the site. Maybe there is a book, a course, or just need to memorize commands from different libraries? This is a very serious issue for me, please help me. Kindness always comes back at times!