The lowest note in standard tuning is an E. 
However there is an open D tuning that is common called DADGAD (that's the notes the open strings are tuned too) which will get you an E flat on the first fret and a D on the open 6th string (lowest string). But now you will have to change the way you play all your chords. 

You can play chords such as D in standard tuning but the root note (d) would be up an octave (on the open 4th string)
Or C with the root on the 3rd fret of the 5th string. This may let you play your song but the bass line in your head might not be exactly as you hear it. 

Another option is to transpose the whole song up to a new key. Meaning the lowest note you're hearing becomes E which is the lowest note you can play in standard tuning, and all the other notes (chords) get shifted up. Example: you are hearing d major and then e minor, change that e major and f# (f sharp) minor. It will have the same harmonic properties but now will be easier to play on the guitar.