A great deal of utter rubbish is talked about Freshers Weeks and usually by people who aren't actually at University.
If you believe all the silly stories it must seem like a week long Ibiza Party Night, with every form of debauchery thrown in. Its actually far, far tamer than this. Yes, there are people who get off their faces on every substance possible and think thats grown-up, but the majority of people aren't like that. Freshers Week isnt all about 'getting drunk' anyway. Most of the week is about introducing you to Uni life and the practicalities of meeting tutors, intro lectures, sessions on IT provision and 'how to use the Library' etc. And there are heaps of social things to do that dont have alcohol as a focus - think of all the overseas students who don't drink at all - and Unis are careful not to promote the 'getting wasted is clever' image and make sure there are general 'fun' things for everyone to do.
Even if you aren't a drinker - go to the main Freshers Ball. Its a laugh, and you can drink coke all evening and no-one will even notice. Join up with a few other like minded (shy) people you meet in the first few days and go together. Leave early if you want to - but go.
Uni is about a great deal more than this first week - its three years, and probably the most fun three years of your life. Enjoy.