My truck squeals when in gear how can I fix it?

I have 2001 Ford f150 and when I start it up and shift it into gear it starts squealing. At first I thought it was the belt but after using a full can of belt dressing I realized that it isn't the problem. Any suggestions would be appreciated

Check the transmission fluid level. If it's low, add some. If that doesn't help, go to a transmission shop.

Have the transmission checked. DON'T just add fluid if it is low. If the transmission is squealing, you have ALREADY damaged the transmission.