Where is Steam purchase?

Excuse me, could you please clarify where exactly I can make a purchase on Steam? I'm a bit confused as to the platform's interface and where the exact location for purchasing items is. Is there a specific section or tab that I should be looking for? Additionally, if I'm making a cryptocurrency purchase, is there a specific process or set of steps that I need to follow to ensure a smooth transaction? I'd appreciate any guidance you could provide in navigating this process.