You need more to go on than simply "I'm going to write a story about the rake". You need to pick a genre, an overall theme and work on the general plot basis, including a rough idea of how it will end. Knowing how a story will end, even if it's as simple as "main character dies at end" can really help you work out how to write the rest of the story. For a diary theme I'd start every entry with the date as a sort of title, none of the dear diary crap or anything. You also need to make it more personal if it's a diary. Perhaps add a note at the start on how/where he bought the diary and add random (But not out of place!) comments about going out with friends or bad days at work inbetween rake investigations.