If you've never communicated with an academic before, best to play it safe and address them either as Dr. Lastname or Prof. Lastname. Be absolutely sure to check which title they use by going on to the university or institution website (professor is an earned title in the UK, and most academics are not professors). If they don't have a PhD (they might be a student) then usual title conventions apply.
Include your full name (including your title) in your message if communicating in writing. If they address you with your first name and/or don't use their title in their response, then you can safely assume, at least in writing and one-on-one conversations, that you're on first name terms now, and should from then on communicate as such. I find most academics tend to prefer first name terms nowadays.
One important thing to note is if you're conversing with them with an audience. Some academics will be happy for you to be causal with them privately or with a group of other academics, or perhaps even PhD students, but they will be bothered if you don't use their title when introducing them or conversing with them with a group of undergraduates listening in, for example.