When i start my car you can hear this weird squealing noise,its definetly coming from the alternator.When i turn my headlights on he noise get louder.How can i fix it!?

Have you checked the belt? If it's loose it will slip and squeal, and the more load there is on the alternator the more it will slip. Depending on mileage and condition, the belt(s) may need to be replaced.

thanks.How would i tell if they are loose.They feel tight?

With the engine off (of course) press down with your thumb at the midpoint of the belt between the pullies. It should deflect a small amount, maybe 1/8" or so. Also turn it over and inspect the inner surface that contacts the pullies, if it has a shiny/glazed appearance and/or a lot of cracks then the belt is shot.
Another thing you may want to do is pull the belt off and spin the alternator by hand. It should turn smoothly and quietly with little effort. If it makes noise, feels rough or binds, or has any play there is an internal problem with the alternator and it should be replaced.

Thanks mullder!