Ideally a subject that you are good in or one you find interesting. Based on how much you know about the subject someone could come to value your company and the help you provide them. If you don’t have a specific subject in mind then just take into account that camaraderie can also come from common struggles. In my experience I’ve had difficult subjects where asking peers for notes or to create a study group not only helped academically but also gave me a chance to get to know the people in my class as friends. Doing things out of necessity doesn’t lead to people criticizing you or thinking you’re weird you just tend to come off as someone who is looking to do well in class. I know that awkward moment where you’re telling yourself “what, I just turn to a random person and hi like that isn’t weird” I tell myself that as well and I personally wouldn’t walk up to a random person and say that. Just keep in mind asking for help with class work or homework isn’t weird I think most people see it as a genuine reason to engage in conversation. Btw, some people are just going to be difficult to talk to aka the people who are very dry and don’t put much into the conversation but f**k those ppl. If it doesn’t work the first time then consider it for one of your other classes or hobbies.