How did you get into journaling?

It's been suggested to me by a few different medical professionals that I might see some benefit with journaling.So how does one get started? I know it's a matter of writing, but I'm a firm believer that it's best to establish a new habit by starting off small and therein lay my issue. What is small? Is it just a quick recap of the day? Are there topics one should include? Does it even matter?