Well it helps if first you are a teacher or in the science research field already.
To get started as a writer in the textbook industry, usually you start as a supplement writer. I suggest you contact the Editorial contact at the academic publishers of your choice, include your VITA and express an interest in writing supplements for the science books. It does pay and gives you a leg up.
You can skip this step if you are have a suitable education background and teaching/research creditionals. Academic publishers need someone who have a background that will garner respect by other teachers in the field. Unless you have strong creditionals, you'll most likely have to hope to be invited as a secondary or late coming author to an established series.
To get your own book, you need a suitable teaching background (or a secondary author to provide that), suitable education, solid creditionals in research, writing or awards in teaching.
Otherwise--supplement author is your best way to get a foot in the door.
This is rarely a full time job but some authors do make a decent living out of it. However, non-teaching authors are often phased out of books--as most instructors want to use a series by someone who has a 'teaching' perspective.
Do you already have a prospectus of a book you'd like to write? It sounds like your background is solid but it would also depend on if your writing style was suitable for college age reading. I also highly suggest you develop a 'unique' twist or feature to sell your book to the publisher. They need a point that makes your book different from all the rest.
Pearson, Cengage, McGraw-Hill are some of the better known academic publishers on the college market. But if you are doing upper level courses, you may have to look at the smaller companies or area specific companies.