How do I buy Steam currency?

Excuse me, could you please clarify the process of purchasing Steam currency? I'm a bit new to this and would appreciate some guidance. Do I need to have a Steam account first? And if so, how do I go about adding funds to it? Are there any specific payment methods that Steam accepts? I've heard of some people using cryptocurrency to buy Steam currency, is that an option as well? I'm eager to learn more about the process and any tips you might have to make the transaction smooth and successful. Thank you in advance for your help!