Circle of fifths
You can use this guide to work out which notes and chords you can safely use in your chosen key.
The key of C can use the notes F, C, G, D, A, E, and B.
The key of C can use the chords F, C, G, Dm, Am, and Em.
The key of C can borrow from the key of Cm the chords Ab, Eb, Bb, Fm, Cm, and Gm.
You can change key in the middle of a song. If you pick a close key, this will be easier to do because they share more chords and notes with each other.
The key of C is most easily pivoted to the key of F, C, G, Dm, Am, and Em.