How to start writing my statement of purpose?
While this will depend partly on the formal requirements of the course for which you are applying, here are some general things that I look for in reading these statements as admissions tutor for our department:


Demonstrated enthusiasm for the subject.  Your CV and transcript will provide basic factual information about your technical skills and ability to succeed in an academic environment, but the statement of purpose lets you round out that profile with memorable detail.  Be specific.  If you've worked at a tech start-up, how did that inform your decision to pursue further study?  What new skills did you learn, new challenges faced and overcome, etc?  
Demonstrated awareness of the programme.  I've read essays that clearly had no understanding of what our programme covered and how we taught it.  Be familiar with the prospectus and don't be afraid to contact the admissions tutors to clarify anything.  How does the style of teaching at your target university relate to your interests?  
Ability to write coherently.  Regardless of the subject, you will need to be able to communicate clearly.  A poorly structured essay, full of grammar and spelling mistakes, is a sure-fire way to end up on the 'no' pile.


Good luck!