To receive a Blue Card, you must meet the following criteria:
Have a higher education. If you graduated from an institute or university outside of Germany, then your diploma must be recognized in Germany. Read about recognition of a diploma of higher education in Germany. If you are going to work as tech specialist (IT industry), then you do not need a higher education diploma, but just prove your work experience (minimum 3 years).
Sign an employment contract with a German company or have a formal job offer from a German employer.
Сertain salary level. See the next answer for more details on what kind of salary you need.
The work must be commensurate with your qualifications. This clause was recently relaxed, allowing you to apply for a blue card even if your higher education diploma is not in the field in which you intend to work.