Most Pythonic solution
# set your list of words, whatever the source
words_list = ('cat', 'dog', 'banana')
# get the word from the user inpuit
user_word = raw_input("Enter a word:\n")
# create an generator, so your output is flexible and store almost nothing in memory
word_generator = (word for word in words_list if word.startswith(user_word))
# now you in, you can make anything you want with it
# here we just list it :
for word in word_generator :
Remember generators can be only used once, so turn it to a list (using list(word_generator)) or use the itertools.tee function if you expect using it more than once.
Best way to do it :
Store it into a database and use SQL to look for the word you need. If there is a lot of words in your dictionary, it will be much faster and efficient.
Python got thousand of DB API to help you do the job ;-)