If you're writing songs for radio etc... it's good to know the different song structures. AABA ABAB etc...
Write about something that's familiar to everybody. Stick to one format through the whole song... like if you start the song in the first person, don't suddenly switch to the third person...
And if somebody asks you what your song's about, you should be able to describe it in one sentence. If you wanna make money, try to make it so the song can cross over more than one genre.. like pop/rock/country, rock/aor/blues etc.. that way it'll get played on more radio stations and will appeal to more people.