1994 Toyota Corolla Not shifting

I was wondering if there is anything that I can do to possibly fix this problem

I have to allow my car to warm up, before it will shift into overdrive. If I do not it stays stuck in 3rd gear and will not shift untill it is warmed. The moment the gauge on the coolant temperature rises above the very bottom line it will shift.

Also, along the lines of the coolant temperature gauge, if I am driving and the gauge goes below the bottom line, my transmission will not go into the more fuel efficient overdrive, or will shift out of it if I had been in that gear at the time.

I don't really know a whole lot about cars, so I apologize if my vocabulary is off.

Thank you