In my case, I always start with some chord sequence in my guitar and when I find something I like, I start to mumble some nonesense to create a melody. When I feel I have something cool that's when I start to put some lyrics.
Something that has helped me several time is to imagine that I am someone else... imagine that you are, let's say, Beyoncé, to name one... you'll see how different ideas and melodies start to appear, just like magic.
Hope you find it helpful
:)