Here's a little history on how this all got started :D
So my club was in desperate need of a new project, and it so happened that I had stumbled across an RC car at GoodWill for about 75 cents. It was here that we figured: "Hey, we can build something out of this." However, once we were knee deep in the project, we realized we had bitten off more than we could chew. However, in the end we managed to swallow and get the project done but it ended up taking 5 weeks and 6 days (41 days) to finish the whole thing LOL. (March 7 - April 17). This amount of time was really just due in part to the fact that the pieces we needed were coming in slowly. One thing that is for certain is that there are a lot of various ways of completing this project. (I hint at the copious amounts of Trial and Error*)
*Lots of errors... -.-
Let’s get to the Nitty Gritty
So first step would be the materials:
3) Wires (Lots of them)
4) L293D (Motor H-Bridge)
6) HY-SRF05 or HC-SR04, it really doesn't matter. I was able to get both of them working just fine
7) 3 x AA Battery Mount or 4 x AA Battery Mount (Go with the 3 pack only because you’re just controlling the Arduino and not pulling or giving any current to the car.)
8) Lots of batteries for testing. I just grabbed a few packs from my local dollar store (Just for testing. Don't use these kinds of batteries for the real fun.)
9) Soldering Iron
10) Optional: Switch (From Lamp)
11) Tape (I used painter's tape)
In my case, my car could pack in 5 x AA batteries. So that's a lot of energy to pack in.
Your going to want to wire everything like in the pictures I've provided.
Note: The pins on the arduino for pins 2 and 4, are wrong. The L293D displays correct pin connections. (Here's to another Error)
Edit: I've also included another picture of the corrected layout.
I've included the .fzz file that will show all of the correct connections for the board.
Modify as needed.