first get a decent arsonal of drums and vsts, then learn the types of synths. you got plucks/keys, leads, pads, bass, atmospheres, arps, synths, fx, trancegates and sequences. In trap you usually use a pluck to make the main melody, which usually is a quarter note run or some variation (i dont really know what a quarter note run is but i hope im right). You also could use a arp but some people frown upon it, then again that doesnt really matter. After that you can honesty pick from anything else, the most common choices would be a lead, pad or bass but there isnt really any clear favorite. For song structure i think you should just decide that on your own, how would you want songs to flow? Its all about layering really, let the beat build. Just put different sound types over one another, lead over pluck over bass/pad over horns/strings. Make sure the melodies compliment each other and dont sound unpleasant.
Dont worry bout mixing yet. Just make sure to not have a limiter on your master track and dont let your music clip. I just say that because i dont like what limiters do to songs and i dont want people starting off thinking they should use them. Its really a personal thing though.
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