How do I send a message to APRS?

Hello there, I'm curious about how one would go about sending a message to APRS. Could you please elaborate on the steps involved in this process? Are there any specific requirements or protocols that need to be followed? Additionally, is there a fee associated with sending a message to APRS, and if so, how much is it? I'd appreciate any information you can provide on this matter. Thank you in advance for your assistance.