These aren't really things I'd say to myself in relation to mistakes I made, but just general advice
Freshers Week, and to an extent the first 4-6 weeks of first year, is not the same as the rest of your time at university. It's totally different. If you don't enjoy the Freshers thing don't be miserable thinking you have three years of it. There are a lot of people at uni which means people with the same interests and personality. You don't need to conform to the mainstream if it's not you, just look for like-minded people.
Choose your friendship group carefully as you are likely to pick up some of their characteristics. If you hang round with people that take their degrees seriously you will probably do well and enjoy each others' success. If you hang around with people that have come to uni to party all the time or smoke weed in their flats all day then you will end up following their lifestyle and it almost becomes unacceptable in the group to study hard or do well because it shows the others up.
Don't spend three years being obsessed by relationships, while the opportunities of uni pass you by. Those 3/4 years you spend at uni present opportunities you will never get in another window of your life: to learn new things, do new things, meet new people, get your career set up. Get those years right and you will be set on a good path for the rest of your life. But so many people look back after finishing uni and wish they'd done something they could have done at uni. IMO the big reason people don't is because those 3/4 years are dominated by relationships and relationship issues. They are spending their whole time thinking about who they fancy, will they/won't they get with them, then when they are in a relationship they spend their time thinking about how it's going, where it's going, the latest argument they've had, then when a relationship ends they spend weeks/months unable to focus on anything other than getting over the relationship until they find someone new they fancy and the cycle starts again. For young people, relationships are obviously important but seriously I saw it happen so often at uni that people lived in a bubble of their relationship issues and everything else, degree, career planning, doing other things they'd really like to do if they opened their mind to it, just don't get a look inside this bubble.