Convert message to polynomial NTRU
What you are looking for is explicitly specified in the IEEE Standard 1363.1 [1], which covers NTRU. In particular, encoding a bit string into a polynomial is done in the following manner: Once you have processed the input message, you divide it into blocks of 3 bits, and transform each block in a pair of coefficients of a ternary polynomial according to a predefined table (described in 9.2.2.h of [1]):


{0, 0, 0} is converted to {0, 0}
{0, 0, 1} is converted to {0, 1} 
{0, 1, 0} is converted to {0, –1}
{0, 1, 1} is converted to {1, 0}
{1, 0, 0} is converted to {1, 1}
{1, 0, 1} is converted to {1, –1}
{1, 1, 0} is converted to {–1, 0}
{1, 1, 1} is converted to {–1, 1}


Note that the input bit string needs some processing before transforming it into the polynomial (also described in section 9.2.2).

Also note that we assume the original message is a bit string, so it doesn't matter which was the codification of the original text string (ASCII, UTF-8 or whatever); that should be a problem of your application, not of the encryption algorithm.

References:

[1] IEEE Std 1363.1 - IEEE Standard Specification for Public Key Cryptographic Techniques Based on Hard Problems over Lattices. 2008.