I always start with the drums. The drum track will serve as a metronome (or click track) to keep the entire arrangement at a consistent tempo and form a framework for all the other instruments that follow.
Since you have a melody in mind, search for a pre-arranged drum pattern that best fits the groove that you want for your song. Adjust the tempo accordingly.
Next I would play the chord progression to the drum track while singing the melody in my head (or out loud if you can record your guitar independently with the tools at your disposal).
Then I would sing the vocals over the drums and the chord progression. Singing the vocals establishes the melody of the song.
From there you can add any other instrumental parts you desire in any particular order.
I might do a bass line next and then perhaps some lead fills or solo's if you allow space in your arrangement for a guitar or keyboard or other instrumental solo.
For the intro (beginning riff) decide how long you will make it and play a chord progression during the intro or just leave space by humming an idea for the intro over the drum track for the first couple of measures (however long you want your intro to be). The drum track will let you know where the measures start and end.
Once you finish all the other instrumental parts and record the vocals, you can go back and tweak the drums, adding fills and changing the pattern for the chorus and maybe add an ending. You can even change the drum track completely as long as you don't change the tempo.