Job interview for H1B while entered USA on B1/B2 Visa
Since you entered US on B1/B2, I am assuming that you came here for tourism.
Legally, you cannot give job interviews on B1/B2 visa.
Your job offer might be viewed as suspicious under these circumstances.
Cap subject H1B
The H1B cap opens every year on April 1. You cannot get your H1b in current year quota if the lottery has already been closed.
Employer files H1B in new season in April every year
Your employer will have to file for your H1B in April for new fiscal year quota.
Stay in US with B1/B2 visa - File H1B and wait for approval?
I strongly advise you to leave USA before filing for your H1B to improve your chances and reduce the visa intention conflict.