I am a 2nd year student studying physics in the UK. I am very interested in securing a research internship at the end of my 2nd year. However, I have not done any research internships in my 1st year, and I'm quite confused on how to write my academic CV(especially what to write in the experience section) for the first time, without prior experience in research. Also how do I email professors to ask whether they are willing to take on summer students? Any advice would be appreciated! Thank you!
Assuming you are just starting the second year of undergraduate, it would be unusual, almost everywhere, to have much of any research experience. Everyone needs to start somewhere, which is what you are trying to do. It might (might) be enough to just say "None yet", but I don't know the requirements of the systems you are applying to.
I would talk to someone local at your university. But it should be someone with some knowledge of what you are trying to do. Whether your tutor has that experience or not, or whether you need to talk to a more senior professor, depends on your university, but find that person. They might even have an opportunity for you.
There may also be summer internships available at some national labs, though I don't know specifics, especially for UK. Those should be advertised and professors may know as well.
But sending blind emails to professors is usually not fruitful, especially if you need funding. But this is another place where a local professor can serve as an intermediary for you, provided they know you and have a high opinion of your progress. They might, at least, suggest someone to contact, in which case you can say "Professor X suggested I contact you...".
Alternatively, you can look for announcements in various professional journals, such as those of professional organizations, of openings. Librarians can often help with such a search.
Your CV, of course, needs to be completely honest, not overstating your accomplishments. People will understand a lack of such for beginning students. But you need to avoid wasting people's time with blind requests having nothing to back them up.