Select the chords you want to use. It's entirely possible to make a song out of three simple open chords. Open chords are easier to play than barre chords because you don't need to lay your finger flat across the fretboard. You may also find that open chords have more resonance, which can add depth to your sound. Begin by experimenting with the chord shapes you are most familiar with. The chords you use will influence the melody, so make sure you're happy with the chords. Consider having two major chords and a minor chord for your verse and three major chords for your chorus.
Practice changing between the chords. Once you have an idea of the order in which the chords go, you should get comfortable transitioning between them.
Experiment with melodies. Strum the chords and hum or whistle a tune over the top. Don't be afraid to try unusual combinations of notes to add a little quirkiness to your song. If you're struggling for a starting point, begin by humming the root note of the chord. So if your first chord is E major, pluck the top E string on its own and hum that pitch. Then strum the chord while humming the E note and progress from there.
Structure your song. You should use the catchiest melody for your chorus. You can use the same chords for verse and chorus or you can change completely for each section; it's up to you. It's traditional for the melody to change in the chorus so that it is distinct from the verse. You don't need to change it radically; consider singing the same melody but in a higher register. Establish how many repeats you will play for your verse and how many you will play for your chorus. Experiment with different arrangements, and if you're feeling really creative, you can put in a middle eight section. This is traditionally a single, eight-bar section that provides relief from the repetition of the verse and chorus structure.
Write down the chord progression. Once you're satisfied with your song, write down the chords so you don't forget them.