it is probably your altenator or AC unit if you have one lossen the bolts that allow it to slide then take a pry bar and stretch the belt very tight hold it and tighten the bolts so it does not move. I just did this a month ago.
It may help to have someone else tighten the bolts while you hold it tight with the pry bar.
Add belt dressing afterwards to help with the squeak.
if after that it still squeaks then your alt or AC could be going bad.