I came to the US on a visitor visa and later got a job offer from a company that is willing to sponsor me. Can I change from a tourist visa to a work visa while staying in the US or do I have to leave the country first

Unauthorized employment can be a concern in many immigration cases, but when someone is applying for permanent residence through marriage to a US citizen, certain violations like unauthorized work are often forgiven if the person entered the country legally. The main focus will usually be proving that the marriage is genuine and that the applicant is otherwise eligible.