The mood of a song is how it tells its story. Your role as a singer is to put a voice to that story, to lift it off the paper and give it life. You are the one who gives that song a place in your audience’s heart.
Listen to the song. Does it make you feel happy? angry? light-hearted? wistful? lonely? grieving? hopeful? uplifted?
Now study what it is that evokes those feelings. Is the song quick and rhythmic, with a strong beat to carry it along? Is it slower, with more of a focus on lovely flowing melody? Is it in a minor (sadder-sounding) key or a major key? Does it switch from one to the other to reflect a change in the song?
Next, how do you sing a strong beat? Most of us think of a march, but most dance tunes have an easily followed rhythm. Short, snappy notes will liven up any song, whereas longer, flowing (legato) notes will smooth out the rhythm and change it more to a ballad.
Flowing, legato pieces have some challenges. Breath control is all-important, so you can sustain passages without interruption. Dynamics are vital, to maintain interest and to help convey the mood. Occasionally the tempo may change slightly as you pass from one phrase or thought to another.
Follow your director meticulously.