have a goal
When I decided to take up running the first thing I did was enter a race. Then I told everyone I knew what I was doing. I am clearly someone who responds well to public accountability because repetitional risk was a real motivation for me! The other benefit of a goal is it gives you something tangible to work towards - you aren't just running for the sake of it, you are running for a reason. It's a motivational tactic I still use today.
If you are thinking about running, what worries you? If you do run, what's your top tip for a newbie?