To best position yourself for a marketing role, you should have an interest and basic understanding in marketing and the connection between a brand and its relationship to the consumers. As a product marketer you manage that relationship so you should be excited about being on both the company side but also understanding and communicating with consumers who inform your overall strategy. To prepare for interviews, its important to understand basic marketing concepts such as STP (segmentation, targeting, positioning), 4Ps (price, place, product, promotion), and 5Cs (company, collaborators, competitors, customers, context). When looking into PMM roles in tech specifically, also understand why you're interested in doing marketing for tech as opposed to a CPG or more traditional marketing route. I applied through the BOLD Internship program that focuses on underrepresented groups in tech. You apply to the internship under the APMM function, and then are assigned to a function and a product as needed.
In general, marketing interviewers want to see that you have high energy, strong communication skills and a pure passion for the subject. If you are applying to a particular agency or brand, get familiar with their work prior to the interview. I find that it impresses interviewers when you name-drop a campaign and can iterate why you find it effective and how you can learn from and contribute to it. Lastly, a veteran in the industry gave me great interviewing advice: always have 3 things that set you apart from the crowd and try to incorporate them into the interview. Even if you aren't asked "tell me about yourself," you can find a way to weave these 3 personal strong-points into your responses.