Music Theory: Need help with chords/song writing?

I have little piece I have been working on…its not much but I need help or ideas with more chords.

I need a very tension chord I think would help to lead into a resolve onto G.

I have G minor, I liked the sound of F5 inverted power chord.

The main melody uses

notes C D D#/Eb F G

I made a notation version which I am not very good with musical reading and all, just a guitar player but maybe it will help understand what I am doing….
http://www.noteflight.com/scores/view/afe8aec0f71fb438e7ea3bbb350cd9b834e40572

Also what is the key signature, I want to say Bb major/G minor would that be right?

If you have any tips or advice on writing/composing I am all ears :)

Thanks.