Afraid you'll goof when writing your songs? There's NO wrong way. Hey, it's YOUR song. Here's a fun hint that might help you find JOY in YOUR music for the World! --
USE ODD NOTES. Be different in *yYyour* way, your uniqueness of ideas!
1. Deliberately throw odd notes into your melody that are not in the scale of your chord. I accidentally left the sharp mark off an F in song Claxton and I recorded last night, FEAR FREE PRAYER (http://www.didoreflections.com/song/fear-free-prayer). While practicing, I tried that weird F note I'd typed into the interlude melody line. It was a really cool mistake, maybe even God-guided?
2. Really weird, unexpected (not in the scale!) notes create a dark mood for dark words like fear, sorrow, sin, or pain. Odd notes add excitement to your song--- avoid ho-hum predictable melodies.