Writing Statement of Purpose As a Bachelor Student (Physics)
SOPs are a rather dreaded part of the application process for many students in the tagged geography, precisely because there is a big gap between the expectations and method of working that students are typically used to. The previous comments and answer are indeed relevant. Given your current situation, I'll add some specific things that you can think about, leaning towards pragmatism rather than the ideal.
(1) The SOP is essentially a narrative that has to sell. (Arguably so is most research, but that's another tale). You must know your audience. Don't make one boilerplate SOP for all the departments you apply to. Visit their websites, read up what they work on. Try to use social media to speak to current students. You may not know what you want to work on, but you must know with reasonable veracity what they work on. Don't start writing your SOP without doing this. The underlying benefit (arguably the real reason) is that you will me more updated about what is currently happening in the field now, and this will help you cultivate your interests more sensibly.
(2) Treat each SOP as an independent and exclusive document. Once you've identified the department &amp; professor/group you would like to work with the most, think of everything you've heard, learnt or done that may be relevant to the topic. Oftentimes this is a research project, sometimes it is a class that really interested you and motivated you to read up further. Sometimes it is even a personal experience or anecdote that left a deep impact on you, inspiring you to move towards the respective field. You need not have done concrete research work on it, but you should be able to demonstrate some interest or passion, or even a convincing motivation.
If you are not able to find yourself motivated by a certain field, it may be a strong indication to look elsewhere - one more underlying benefit.
(3) Connect your background (educational, personal if it is relevant), your motivation/experience/interest and relevant achievements to the field of interest. Avoid irrelevant history. The badminton championship you participated in is unlikely to be helpful, unless you are using it to make a point about striving for excellence. Or being fascinated by the aerodynamics of a weighted shuttlecock. I hope you catch my drift here.
(4) Take seriously the last segments, which describe how the university could help you achieve your goals, and equally, why you are a good fit for the university. Be very specific about the first. You should be able to demonstrate that you know what the department/group is known for, what sets them apart, what sort of impact they have. Use details if possible. If you've been able to speak to some students there, mention it (show your efforts and initiative). As to why you are a good fit, describe your attributes and qualities; but ensure that they relate to the personal history/experiences/achievements that you previously mentioned. This adds continuity, keeps you honest and prevents the SOP from reading like a boilerplate job application.
Take your time with this, and remember to respect the reader. Its a big opportunity for you and an important investment for the group/department that you join. The worst thing that you could do is treat it flippantly and churn out something that doesn't show serious thought.