star - Conversion from Equatorial Coordinate to Horizon Coordinates

It depends from which celestial coordinates you are converting. I suppose you are working in equatorial coordinates. Then, you can convert as following:

$$tan A = {sin h over cos h sinphi_o - tandelta cosphi_o} $$ $$ begin{cases}
cos a sin A = cosdelta sin h \
cos a cos A = cosdelta cos h sinphi_o - sindelta cosphi_o
end{cases}$$ $$sin a = sinphi_o sindelta + cosphi_o cosdelta cos h$$

Where $A$ is azimuth, $a$ altitude, $alpha$ right ascension, $delta$ declination, $h$ hour angle and $phi_0$ the observer's latitude.

Source: Wikipedia