Old tile shower with missing grout
I apologize, I initially posted this in the wrong section!
My pink tiled shower is missing grout in a bunch of the wall tiles. What do I need to do before I attempt to regrout the wall? I know water has gotten behind the tile. I pried one tile off the bottom row (directly above tub) to see what was behind it, but I have no clue what I am looking for! I can upload a picture if needed.
The tile, and probably the grout, is original to my house which was built in 1957. The walls in my house are plaster, not sheetrock.