Plumbing?? ? Please help! I have very low pressure in my shower but is fine everywhere else in the house.?

It started with the cold water valve getting more and more difficult to close fully. After a while it just wouldn't close all the way so I turned the water off, then I took the valve for the cold and the hot into a local plumbing store ended up replacing one of them and replacing the washer on the other. I put them both back in and turned the water back on. I had really low water pressure in the shower after that. Someone said to turn the water off, open the valve all the way and then turn it back on. I did this multiple times but with no results. My pressure is so bad at this point that I have to put a plug into the pipe coming out of the wall where the spout is to get it to go up to the shower head, using the diverter doesn't work. There is also a steady drip of water that comes out now so I assume I must need to readjust the valves at least somewhat although I don't see exactly how I could have messed it up. I have one pressure, whether I turn the cold one half turn, or fully open, whether I turn the hot one half turn, or fully open, or I open both partways, or fully open. Here is my question, do I need to just start replacing some of the pipes leading to the shower or is there something else I can do? It is an older house with galvanized pipes. If you think that the only option is to replace the pipes, do I replace with galvanized or switch over to copper or something? What do I need (tools and materials) if I do this? I don't want to spend a lot of money so that is why I am going to do it myself even though I have never done any plumbing before. Also, with my 2 work schedules, the only day I have free to get into it is this Thursday (Thanksgiving) so I need to make sure I have everything in advance since no stores will be open. Any ideas/advice would be appreciated.