You need to contact your local Immigration office. You need an endorsement from your embassy of natural origin. It would then be deferred to an INS officer for review.
The time to start doing it is before your B2 expires. Go to Immigration and notify them of your intentions and fill out the paperwork to start the process.
The best and easiest possible way to convert the B2 Visa to an H1B Visa is to leave the United States of America and apply for a h1b visa from your country using proper documentation required for the h1b visa.
