C# major's relative minor key is A# Minor. These two keys both contain the same notes but a different tonic (in this case the tonic being C# and A#, accordingly)
C♯(i) D♯ E♯ F♯(iv) G♯(v) A♯ B♯
A♯(i) B♯ C♯ D♯(iv) E♯(v) F♯ G♯
This doesn't really have any bearing on how you write a song in a major key, but it does help us to understand the relationship between them.
To put it very simplistically, to write a general "pop" song in C# major you will root your melody (riffs and hooks) and harmony (chord changes) around the C#. This is usually manifest in starting and ending on the I (home) chord of C#maj, with support from the IV and V chords of F#maj and G#maj. This should sound kinda happy.
To write in A# Minor you use all the same chords and notes, but rooted around the A#. So try starting on the I (home) chord of A#min, with support from the IV and V chords D#min and E#min. This will sound kinda sad.
Once this basic idea starts to make sense, throw some extra chords in to each framework, remembering to anchor around the I, IV, and V.
C#maj(i) D#min E#min F#maj(iv) G#maj(v) A#min B#dim
A#min(i) B#dim C#maj D#min(iv) E#min(v) F#maj G#maj
This is a very simplistic explanation which I'm sure will not hold up to any great scrutiny, but I offer it up as a basic answer. Full disclosure, I'm not a trained musician, this is how I understand these things to hang together.