By being a human you'll constantly think. Write your thoughts down. There you go, you know now what to write about!
On a more serious note, look for anything you are interested at and start to write it. You don't need to be a professional, you can write about it as an amateur and share your journey with others in a blog post or Twitter. Hell, even facebook.
Let's say you want to learn a new programming language. You'll have many thoughts like: hey, this function is pretty neat. This and that. I like this, I don't like that.
Find a clever way of writing those thoughts that is concise and that people would like to read.
If you are too shy and don't feel like sharing your thoughts with anyone, write it for yourself. Imagine if you could open a box with your texts from 1 year ago, with how are you feeling and what you are worried and excited about. I'd bet you would be interested at it. But you don't have it now. So start stacking those texts now for the next year.
As for exercises, there are so many and they will improve your writing in different ways, it's up to you to decide what is important. Find anybody that is inspiring to you and see how they write and communicate.
Also, like any art or subjective matters, writing is all about observing. Our brain has too many things to care about. If you don't make the conscious decision of looking at a text with a observer POV, it will just be yet another text and your brain will only focus on its content, not so much with style.
