What you are thinking about doing matters more than you might realize. Inmates who have outside contact, people who write, who check in, who treat them like human beings worth communicating with, have measurably better outcomes than those who are completely isolated. A letter from someone who has no obligation to write but chose to anyway carries a different kind of weight than almost anything else that comes through mail call.
Getting started is straightforward. InmateAid has over two million incarcerated individuals in its database. You can browse and select someone to write to, then use the Letter to Inmate service to compose and send your first letter. There is no complicated sign-up process and no requirement to commit to an ongoing correspondence before you know if it feels right.
A few practical thoughts for starting out. Keep the first letter warm and simple. Introduce yourself, share a little about your life and your reason for writing, and ask questions that invite a response. You do not need to have a perfect letter, you just need to show up on the page as a genuine person. That is what makes the difference.
You do not need to share your personal address. InmateAid's return address appears on everything that goes out, which means you can correspond freely without your home information entering the facility's system. That privacy protection lets you engage at whatever level of openness feels comfortable to you.