I'm starting uni this year Inshallah and I it will be mixed. I've haven't interacted with guys much unless it is necessary bec I was mostly at an all girls schools. How should I deal with guys knowing that I don't like it when they approach me and they mostly do it to befriend and not for necessary stuff?
That depends on your personality and culture, some women are self-confident and can easily end unwanted interactions. In general just being professional and polite is going to be good enough for most situations. If you wear the hijab, then that in itself is a big signal that you do not want to be approached the way other women are. Something that could help prevent unwanted interactions outside the classroom is to wear sunglasses. People will be a lot more hesitant to approach you if they cannot see your eyes. Another thing would be to wear headphones.
If you get persistently approached or harassed, performing a regular spiritual practice such as tahajjud is going to help greatly in remaining calm and undisturbed in difficult situations.