There are dozens of ways to start.
If you play nothing but can hum the melody or sing it then you could download Songsmith which allows you to enter a melody via a mic, guitar, keyboard etc & it will generate a backing arrangement with a range of specs you give it. It's a little crude but will give you the chords that you need to accompany your song.
band in a box may help there are a squillion way in.
I know a bloke who couldn't play but developed his own notation system for HIS voice and he successfully wrote, then with the help of friends, arranged and produced 2 or 3 quite successful amateur musicals.
Personally, I don't "Sight Read" but can read music to some degree, (bass clef funnily enough but it takes time & energy for my poor brain to translate into doable stuff), but I also don't have a very good ear so can't play just by sound. I use chords as the basis for building an arrangement & those doz chords I know get a work out.