Deconstruct what you already know into smaller parts until you reach "atomic" knowledge (i.e. things that cannot be reduced into other factors). At every step ask yourself "Why is this the case?". Do this enough and you will probably find interesting connections in mundane knowledge as well as the borders of current knowledge
Also if you aren't an expert in your field, what you will want to write has probably been written before. Don't fall prone to your ego or the thought that you have something to share that others couldn't teach as well. Use this to focus on things you find interesting or important and make it easier for others to be interested as well