Way behind? Based on what you said, you are just fine. The majority of students in my freshman classes had no programming experience. Hell, I had no programming experience until my sophomore year (changed majors) and I'll be at Google this summer.
If you have no experience with data structures and algorithms, I highly encourage you to read a book first. One book that really helped me is the Skiena Algorithm Design Manual.
If you really really want an internship freshman year, read an algorithms book (see answer to question 2) and start solving leetcode and hackerrank problems. These two sites have been the most useful for improving my interview performance. Eventually you will recognize patterns that occur in many problems and thus be able to solve them.
See my answer to 3 lol. You can definitely practice this stuff outside of those sites, but they offer a great opportunity to practice since they often provide a large number of inputs and test cases. (if you were to do the same on your own, you would have to test your own inputs and test cases, which may end up wasting your time).
Since you are just in highschool, i would recommend ignoring this and then reading this at the beginning of your junior year. However, since you are interested in big4 internships your freshman year, you might want to start now. If you are really talented/smart you can start in the fall and still get summer internships at top companies. Just delay your interviews until you are comfortable.