I am now entering the field of project management and I am reading on google and other books I have found. My question is what is the difference between Project management vs. Program management vs. Portfolio management? because I am very confused as I read them...
Using a gross simplification, the main difference is size.
You need to be aware of the definitions for project, program, and portfolio. From the PMBOK:
Project: A project is a temporary endeavor undertaken to create a unique product, service, or result.
Program: A program is a group of related projects, subsidiary programs, and program activities that are managed in a coordinated manner to obtain benefits not available from managing them individually.
Portfolio: A portfolio is a collection of projects, programs, subsidiary portfolios, and operations managed as a group to achieve strategic objectives.
You use project management skills to manage all of them, but as you can imagine, managing one project is simpler than managing one program, which in turn is simpler than managing one portfolio.
Here is a good article further explaining the differences.