What do I need to fix now
You have tools?
"What you need" for electrical troubleshooting:
A 12 v test lamp
A multimeter
Some "clip" leads. You used to be able to buy them at RadioSh$$t. I recently ordered some from Mouser Electronics
And...........a FACTORY shop manual. Thanks to some of the guys on here, you can download them free..........over at MyMopar.........The 73 is two sections, get them both
http://www.mymopar.com/index.php?pid=31
The factory diagrams were a bit convoluted in 73, so also get the aftermarket diagrams, which are simplified but not always complete
http://www.mymopar.com/downloads/1973/73ValiantA.jpg
http://www.mymopar.com/downloads/1973/73ValiantB.jpg
Clip your meter to the starter relay "push on" terminal which has a YELLOW wire going to it. Twist the key and watch the meter........see if you have battery voltage there.
If so, clip your meter to the remaining "push on" terminal. Don't disconnect anything, probe them from the back with the wiring still hooked up. This time, when twisting the key to start, you should have VERY LITTLE voltage, which correctly shoes that terminal is going to ground. That wire goes to the neutral safety switch (auto) or clutch switch (which you have to depress) with a stick shift car