Newbie Question about sequence multiplication

[Scott]

Alright, so I've been trying to teach myself Python which, when compared to
my attempt to learn C++, is going pretty well.
But I've come across an issue that I can't figure out, so I figured I'd ask
the pro's.

Now it looks pretty weird in this format but it was copied exactly from IDLE

*****code follows*******

#What this program is suppose to do is print a sentence centered in a box

sentence = raw_input('Sentence: ')

screen_width = 80
text_width = len(sentence)
box_width = text_width + 6
left_margin = (screen_width - box_width) // 2

print
print ' ' * left_margin + '+'   + '-' * (box_width-2)  +  '+'
print ' ' * left_margin + '|  ' + ' ' * text_width     + ' |'
print ' ' * left_margin + '|  ' + ' '   sentence       + ' |'
print ' ' * left_margin + '|  ' + ' ' * text_width     + ' |'
print ' ' * left_margin + '+'   + '-' * (box_width-2)  + ' |'
print

****end code****

Now that, even though copied straight from "Beginning Python: From Novice to
Professional", returns :
There's an error in your program: invalid syntax

with the word sentence highlighted (not the sentence when I'm defining the
name, the one in......uhm....the body of the code)


Now if i put * before sentence as it is with the rest of the variables, it
actually gets to the point where it asks me for the sentence, but after
inputting my sentence I receive:
Traceback (most recent call last):
  File "D:/Programming/Python/sequence string multiplication example", line
16, in <module>
    print ' ' * left_margin + '|  ' + ' ' * sentence       + ' |'
TypeError: can't multiply sequence by non-int of type 'str'

Why can't I get it to do what it's supposed to do? What am I
missing/misunderstanding?
Very simply all its supposed to do is something like this (now bear with me
formating might distort this a bit lol)
+------------------------------------+
|                                                        |
|                      Like This                    |
|                                                        |
+------------------------------------+

Any help would be greatly appreciated


-Scott