One of the activities I like to do before I actually sit down and write is "clustering." It jostles the mind, loosens up ideas so they float up and you are able to catch them, as if casting a net in the sea and capturing fish. What you get are seemingly random ideas/thoughts. Welcome them. Again, there's no right nor wrong here. Think of "clustering" as playing, and then you're on the right track. Don't "think." Just let it rip.
Make a circle in the middle of a page, and write a word in it. Usually this word should be related to what you want to write about, but if you don't have a topic as of yet, you can jot down the first word that pops into your head. Any word, no matter how silly.
Now, without thinking, "catch" the other words/ideas/thoughts that surface. Draw a line to the main word, create a circle and write that word inside that outer circle. That word will bring up other related words/ideas/thoughts - write those down as well to "catch" them. One thought spurs on another thoughts, and so on.