What are the necessary requirements for coding decimal to binary [Python]?

First of all, welcome to StackOverflow. It looks like you've been burned on questions in the past, and probably rightly so. This site is not typically well-suited for rank amateurs, who are better suited looking up a tutorial or reading documentation.

That said, you seem earnest in your efforts and have phrased the question in a meaningful way, so let me try to help:

1) Need to have a raw_input so the user can type in their number

That's correct, the way you store user input in Python2 is raw_input, as such:

variable_name = raw_input("Prompt")

2 and 3) Input validation

Basically both these points are the same -- how do I make sure my user entered data that's appropriate? There's lots of schools of thought on this, and you should read the difference between EAFP and LBYL coding, as well as concepts like duck-typing and etc. For now, let me just show you what I would do.

number = raw_input("prompt")
try:
    number = int(number)
except ValueError:
    # the user entered a value that can't be converted
    # to an integer, e.g. letters or a decimal number
    # so you'd handle that here, maybe with...
    print("Invalid number")
if 1 <= number <= 255:
    # user has entered a number below 1 or above 256
    # handle it as above, probably ending with:
    return

4) Conversion and return of value

Once you've done all the steps above, your number is GUARANTEED to either have thrown an error or be an integer between 1-256. From this point on, it's smooth sailing, just return the binary representation of that number (which is done with the built-in bin)

else:
    # number is now an integer between 1-256
    return bin(number) # bin(x) returns the number in base 2

Clarification

There are some terms here you've never seen before, so let me explain.

try:
    do_a_thing()
except Exception:
    handle_exception()

This is called a try/except block, and you use them when you're expecting the thing you're doing inside the block might throw an exception. For instance I might do:

try:
    f = open("path/to/a/file.txt", "r") # open a file in read mode
except (IOError,FileNotFoundError,PermissionError):
    print("Can't open that file because of an error")

I know when I try to open a file, it might fail if another application has it open, or if I'm not able to access the drive it's on, or even if the file doesn't exist! I then specify underneath how my code should handle each situation, and could even do

try:
    f = open("path/to/a/file.txt","r")
except IOError:
    handle_IOError()
except FileNotFoundError:
    make_file("path/to/a/file.txt")
except PermissionError:
    get_permission('path/to/a/file.txt')

To handle multiple errors in different ways.

Final Product

If this were my code, I would write it like this:

def get_binary(value):
    try:
        value = int(value)
        assert 1 >= value >= 255
    except ValueError:
        raise TypeError("Value must be an integer")
    except AssertionError:
        raise ValueError("Value must be between 1-255")
    return bin(value)

user_in = raw_input("Enter a number between 1-255: ")
print "The number in binary is {}".format(get_binary(user_in)[2:])