Newbie learning Astronomy and was curious how to locate a star giving a Latitude and LST. For example if my LST is 4:00 and my latitude is 20*N, how would I find where the Horsehead Nebula appears in the sky? The only formula I can think of is the Hour Angle formula..
Welcome !. I think you still need the co-ordinates of the star that you are interested in, its Right Ascension (the sky equivalent of longitude on the Earth) and the Declination (equivalent to latitude). Then as you say, you can use the hour angle (HA) formula:
HA = LST - RA
The Hour Angle tells you how many hours west or east of the meridian your star is (and can be further converted into an azimuth).
For the altitude of the star, you need a bit of trig:
altitude = arcsin(sin(latitude) * sin(declination) + cos(latitude) * cos(latitude) * cos(HA))