What are the steps required for a Croatian to gain a tourist visa for USA? I really want to visit San Francisco and Los Angeles by December this year and I have no clue what to do or where to start.
You will need a Visitor Visa. The linked page describes all the steps you need to apply, and what information to submit in your application.
Unfortunately Croatian citizens are not eligible for the Visa Waiver Program (VWP), which makes this process a lot easier for citizens of certain countries.