How to learn the first bit of m in padded RSA
I assume that this is a homework problem, and so I won't give you the answer.  However, here are a few hints:


What does 'it is a valid ciphertext' mean?
If $c$ decrypts to the value $m'$ (not counting the depad operation; that is, if $m' = c^{1/e} \bmod N\ $), what does $2^ec \bmod N\ $ decrypt to?  What is the relationship to those two values?