read first line from sd card and save the position for again reading from sd card
You should not need to save the position, the file knows where you last read from. You'd only need to manually save such information if you close the file and re-open it again, which unless you have a good reason is actually a very wasteful operation.
If you really do need to you can use the .position() and .seek() functions:
Save the location:
uint32_t pos = myFile.position();

Recall the location:
myFile.seek(pos);

Documentation:

https://www.arduino.cc/en/Reference/FilePosition
https://www.arduino.cc/en/Reference/FileSeek