If you want to learn how to create melodies, there's one piece of music theory that will really help you and a lot of pieces that won't. You should focus on learning a major scale- and ignore the other theory for now. You don't need to know counting or chord progressions or any of that stuff to write simple songs. Just learn the major scale and play notes and chords with notes from it.
The easiest way to do this would be to just compose only using white keys- the C major scale. If you start just making songs with chords which are only white keys and notes that are only white keys, they will not sound hideous. I can't guarantee they'll sound good, but it'll be much easier to get to sounding good than if you try and use the black keys as well.
I'm not familiar with the music of Depeche Mod, but I know that most pop songs you hear on the radio use mostly major scale notes in their melodies and harmonies. It's part of what makes them all sound the same- and all sound consonant.
So if you want somewhere to start, I'd suggest starting there.