Nobody will care if you have sex in Freshers Week.
Where you can get a bad reputation is if you sleep with two or more boys from your halls in the first couple of weeks. Once the guys start to get to know each other they will all talk and will be discussing in detail what the girl is like in bed, what she does, if she takes it up the wrong 'un, if she deep throats, if she swallows, does she do it bareback, all this will be discussed by the boys in halls so your sexual behaviour will be known about by the other boys. Having said this they aren't going to be 'judgemental' about it, they will just think it's funny/sexy or whatever, and won't treat you any differently.