Step 1: Decide the title and topic of your song. Decide what you want your song to be about. Songs can be about anything. Most singers try to write songs about topics that their listeners will resonate with. A LOT of songs are about love - ... or breakups - because many people experience this and thus will recognize themselves (or their stiuation) in the lyrics (...often making the songs very popular).
However, not every song has to be incredibly deep. Sometimes people just write about something that happened to them (or someone they know), that they saw or that they are interested in. Songs can also be educational, like songs about history or math.
Songs can literally be about anything: it's a way of expressing yourself. It's an art form!