All You need is Water, Tammerind ( Chintha Pandu), Turmeric(Pasupu), Salt, Asafatoida(Inguva/Hing), Sugar or Jaggery, And for tadka(popu) Chilli, oil, Jeera, Methi, Mustard.
To make this Add the tammerind, turmeric (Small Qty), Hing, salt to the water and boil it. donot put toomuch of tammerind. By chance if you happen to put more and the rasam is sour, add little sugar or jaggery and boil it. This reduces the sourness of rasam.
All the taste in the Rasm can be enhanced by the tadka. While putting tadka, wait till the jeera and mustard are completely fried. jeera turns white, and mustard breks if the tadka is ready. Just add the tadka to the rasam and serve. (keep your face away from the rasam while putting tadka. normally the hot tadka when it is put in the rasam it spills around. )
The Rasam has varients like Tomato Rasam, Kothimera Rasam, Mango Rasam ( My wife makes this varient very tasty).