I have couple of more things to add if Andrew don't mind:
- First thing is to check out all Andrews theory lessons, specially the ones that relate to chords and how they are derived from scales, this will give you a clear insight to what chords can be used within a key.
- you can check out my songwriting lesson for some useful tips about songwriting
here- last (but not least important) is the fact that good song mostly resolves around good vocal melody, or good main lead melody/riff. If you want to make a good song, I suggest you focus a lot more on vocal melody first, then try to make the lyrics fit in, and THEN you can make any kind of backing in the background - symphony, regular chord acoustic bashing, metal song, rock song, doesn't matter. If the CORE is good, the whole song will be good. If the core (vocal/lead melody) isn't good, no production or arranging tricks will make it better. As with everything today, idea is the most valuable thing today, since everything is pretty much made. What isn't made is what you can focus on and make something interesting that will "catch" in listeners head. It may seem simple to make a simple catchy melody, but this can be very hard.