Hi mate,
I would advise you to learn a few basic guitar chords. G, D, C, F, Am, Em, E and get used to playing them in different chord sequences and at different tempos. Mess about with some chord progressions (even if they don't work at first). Once you have a good idea of what the chords sound like, start applying them to your songs.
I find if I start playing random chord sequences and humming (or singing rubbish lyrics) helps me to get the melody. Then I can start getting a uniform melody that I can apply the lyrics too. You can play the same 3 chords yet have a million different interpretations of how the lyrics would sound. Finding the right combination will be the key to a great tune. Once you get the 'bones' of the song you can start adding all the extras like backing vocals, guitar solos etc.
This is only my personal method and I am sure there are a lot more pros on here who can give you better advice. Listen to everyone and take the bits that woek for you.
Hope it helps nd good luck with the songwriting.
Tommyboy2