Generally when the melam is done the base or adhara sruti is 2 (D). and for normal practice the sruti is set to either 2 and half (D sharp) or 3 (E), to set the sruti to either D sharp or E you need to get number 30 string for Sarani. You can go to any music shop and ask for Number 30 string (for sarani) and the corresponding other strings.
The tala strings ie., SA, PA and SA will be always determined by the main sarani string. So if you putting 30 for Sarani, then the talam strings will be 31, 30 and 30.