Science writing can be lucrative work with publishing opportunities in newspapers, magazines, trade publications, broadcast media, newsletters, universities etc etc.
The best place to learn science writing is to read it! Identify a style you like and try to emulate it. Good science writing is objective, well researched and has a high level of content.
There are much easier ways to make a living if you are suitably qualified than through science writing. Many people choose science writing as a career as they love it and would rather be fulfilled than earn a massive salary.
Know what you are talking about! If you are writing for a scientific journal be aware that your readers may know more than you about the subject. If you don't do extensive research you can expect to be brought up on your shortcomings! Building respect amongst the scientific community who are, lets face it, sticklers for details is notoriously difficult.
Pick an area to specialize in. Science writing can cover an enormous breadth of topics. Pick one or two and concentrate on getting to know everything and everyone connected with them before writing your articles.