You'd use a form in your web page similar to the one here:
<div id="myForm"><form method="post" action="car_script.php">
Make: <input type="text" name="make" /><br />
Model: <input type="text" name="model" /><br />
Year: <input type="text" name="year" /><br />
Miles: <input type="text" name="miles" /><br /><br />
<input type="submit" />
Of course you should do what ever layout or styling you need to do in your style sheets to make work for your site.
In the action element of your form tag you call the script that creates the page that you want to display after the form data is submitted. This includes sending the data back to the same script that created the page that hosts the form if you want to. Also, within that page you can process the data to save to the flat file that you were speaking at the same time if you like.
You can use the data within the page that you are displaying like this:
The make of your car is: <?php echo $_POST["make"]; ?>!<br />
The model of your car is: <?php echo $_POST["model"]; ?>!<br />
However, it is highly recommended that you "validate" your data before doing anything with it because some clever folks could load scripts into your text inputs and make a mess of your stuff for you if you don't. Google data validation for php and see what you can come up with.
Hope this helps point you in the direction you want to go.