Funnily enough, mine was doing this recently, Had a new belt fitted at service around six months ago, and over the last couple of weeks it has been squealing on cold start-up. I removed the plastic cover over the alternator pulley, and then it's just a case of loosening the alternator retaining nut and winding in the tensioner bolt a few turns to give the belt some more tension.
Was a two minute job with a 10mm and 13mm (I think, might have been 12....) socket