aneniine
You might want to think of a theme you are going for, too! It could be a love song, a sad song, something that fits you the most at the moment of writing!
You don't have to write a full song in a row also, you can start, for example, writing down random lyrics you have come up with or try to write the chorus part and then put everything together.
As for the music, you might want to start with something easy, like I'm playing guitar, I would just put together a few chords and then see how it sounds. Then I would start to think of a melody and then I would make the melody different for different parts of the song. Then I would add some cool effects to it. 
Hope this helps!
Posts by aneniine
I love any Ed Sheeran song! Listening to him now! And also Sweet Thing, love that band.