A sketchbook is like diary for an artist, a place where you can just let out all your art without any worries. Any person who wants to get better at drawing should have a sketchbook, because it not only records your ideas, thoughts and experimentations, but a sketchbook also gets you drawing and drawing, building your skills with every moment.
You will see that the quality of your sketchbooks will be better and better, and what at first was dull then will become almost a work of art for itself.
In a sketchbook you can do anything you want, although some artist prefer to have sketchbooks based on themes. For example you could have a sketchbook only in human anatomy, and another just a “test sketchbook” where you try any kind of ideas that comes to your mind.
And remember that sometimes is not all drawing on your sketchbook, you can write some ideas too, you thoughts, all this is valuable when years pass and you can have a better perspective of how your art has evolved.
What can you draw? That is up to you, but I know sometimes is hard to just start drawing. And this trivial things often end up taking more time than they are supposed to. Luckily, there are websites and books where you can get some ideas on what you can draw in your sketchbook.
About techniques, don’t hold back. This is personally what I like most about sketchbooks. Since sketchbooks are personal, not meant for the public to view, you can experiment as much as you like. That applies to style, medium, composition. This way you can truly discover what can be done to make your art better. Just go wild would be the better advice.
Consider your first sketchbook as a beginning of a journey, just imagine all the things you will learn on your way, through a road that is full of surprises. Let freedom and creativity be on your sketchbook and have a lot of fun!