transmission problem?

I have a 1995 Buick Regal Custom. When first starting car and shifting to drive the car wont move . then shift to low gear d1 for 20 seconds then back to regular drive car works normally. My suspicion is overdrive sensor or tranny shift control. Where is this found? in the ecu, internal tranny or an
external control. Would someone please help me out.

Try changing filter and fluid first.