Whatever level you are at; I would say for certain the best tool you can get is a capo. That way, whatever chords you do know you can maximize by moving the capo around to match whatever sounds the best with your voice.

This is one of those things that is a little tough to cover over a forum. To hear what you are trying to work with would be ideal so I could better communicate some ideas. If you feel comfortable posting what you have that would be ideal, but I understand if not.

As others have mentioned, select chords that compliment the mood of the song you are trying to write too. Go with what feels good and again, use that capo as that will help find what chords sound best with your voice.

Good luck!