I am on a tourist visa and my friend offered me a full time job. Can I start working immediately if I switch to a work visa

No, you cannot work on a tourist visa. You must have proper work authorization, which usually means your employer files a petition for a work visa like H-1B, and you receive approval before beginning employment. Working without authorization can create serious immigration consequences.

Not until you are able to get a work visa

You can only work full time on a work visa or if you get a work permit. Tourist visas don't allow any form of employment