You need to use the in
operator. Use if first in letter:
.
>>> letter = ["a", "b", "c"]
>>> word = raw_input('Enter a word:')
Enter a word:ant
>>> first = word[0]
>>> first in letter
True
And one False test,
>>> word = raw_input('Enter a word:')
Enter a word:python
>>> first = word[0]
>>> first in letter
False