1994 Isuzu Rodeo Auto Trans Problems?

I have a 94 isuzu rodeo 4wd and automatic transmission. My check trans light is always on and i have to shift it manually from low to 2nd and 3rd. If i leave it in drive it never changes gears and i have to floor it to get it to go anywhere. I've searched online and I might need to change the trans filter and fluid. I also saw that it might be that the alternator belt needs to be tightened. Maybe that's why it squeaks when i start it. But i don't know for sure> can anyone help me?