Raising point at the East horizon is called 'LAGNA' or 'ASCENDANT'.

The point at which ecliptic rises from the eastern horizon that determines the first house in the horoscope. Find out in which charan of which nakshatra is that point rising at the east. According to the vedicRashi chapter determine the vedicRashi of that point. That rashi number is written in the first house of the  horoscope.

Previously we have seen that there is a big question of measuring a circle. From which point we should start measuring it ?

In the western system they measure the longitude of the point from the vernal equinox  i.e. the point at which the sun crosses towards the north. That point is not fixed for each year because it recedes with respect to stars. This phenomina is called Ayan-Chalan.

We, in India, measure the longitude of that point from the star in the chitra nakshatra named SPICA (ALFA VERGINALIS) +180 degrees. That point is the starting point of the Ashwini Nakshatra. The difference between these two points is called as Ayanansh.

So if you use ephemeris as per the western system then subtract the Ayanansh from the position of the planets you will get the Nirayan position. USE ALWAYS THE NIRAYAN POSITIONS.  

For calculating which point is rising at the east, at the given time and at the given place, the Local Siderial Time is used. For that you can refer any astronomical Books.

Now find the Nakshatra and the charan of that Nakshatra where this point is and find the RASHI of that Nakshatra-Charan. That Rashi is the Lagna-Rashi.