The only undergraduate-level book that tries to teach string theory that I am aware of is by Professor Zwiebach at MIT:
https://www.amazon.com/dp/0521880327/?tag=pfamazon01-20
There is an MIT OpenCourseWare University course that is based on the book, using the first edition from 2007:
https://ocw.mit.edu/courses/physics/8-251-string-theory-for-undergraduates-spring-2007/index.htm
However, even this is a bit of a misnomer because the 'average' undergraduate senior at MIT is likely in the top 1% of all students in the country.
You need to have three years of college-level math and physics at a high level (as in MIT level, hardcore stuff) to have a reasonable chance of understanding most of what is in the simplest textbook on string theory. Even that is a slightly watered-down version of the theory.
Professor Zwiebach taught an online course on quantum mechanics that was pretty challenging - it was actually at the level of the real course taught at MIT.
If you are a second year student in college you *might* have a reasonable chance at understanding physics at that level.
I fear that if you try to dive into string theory you would be like a JV high school student who gets tackled by a NFL linebacker. You are not ready for things at that level if you just graduated from high school. You will most likely get crushed.