Something to start with
I should admit that I have never written a in a notebook, or any kind of personal diary; I just don't see any need to do that. So, there's a first time for everything. Thanks to italki for having provided some writing ideas for this topic.
A funny experience, you say? I have had some. One such experience happened to me just before last Christmas. I was driving a car [If it was your car, say, "driving my car"] on the way home when suddenly Santa Claus appeared from nowhere and told me to stop the car. Of course I stopped immediately; who wants to have problems with Santa right before Christmas? Santa came closer and only then did I realise [Note the inversion!] I had [past perfect tense] been stopped by a police officer in Santa's clothing. [Variant: ... by a police officer dressed as Santa.] Fortunately, he was in a good mood and forgave me my slight excess of speed. I was smiling all the way home.