in the most general and simplest case, when u have directly the numerator and denominator, its just
tf(b,a) where b is the numerator vector and a is the den. vector.. if u have to find the frequency response of the same,, then its freqs(num, den, range).
and i sugges u read this chapter "6" of "electronics and circuit analysis using matlab from CRC press....
no explanations can match that.. u could very well start directly.