I just don't know where to start.
Books have been written on this subject I suggest you start there. You are being asked to write the whole song not just a solo.
Slow down and do what you instructor tells you and by the end of the class you will be a songwriter. I'm listing a dirt simple big picture of what a songwriter takes into account...
Will there be lyrics? If so you need a story worth telling. If no lyrics you need a head worth listening to. The story, or head, is what you build on. If this is an instrumental piece you build from the repeated head. If this will have lyrics, you build your song from the story line.
You mentioned that one of the songs should be 12 minutes long. I do not believe I have ever seen a lyric based song that was 12 minutes long. Looks like your instructor wants to see an instrumental of some kind. Moving on.....
My music is sung thus there are lyrics so I start there. I start, by putting the story to be told into verse format. Under a cookie cutter chord progression in the key I sing in. I sing best in D so my key would be D. And the basic D-G-A-D progression using a four line verse format would get me started. Three verses and a chorus will be needed for your first draft.
The melody comes from the key of D and follows the chord progression. The chords are placed so they provide movement from rest to tension, to climax and then back to rest. Each chord should share some of the melody's notes, as this in addition to providing movement brings harmony. Harmony is kinda important.
The bass is a bitch to write songs on. Go get your guitar, or keyboard, and strum the progression while, 1) singing the lyrics while fleshing out your song, or 2) building your instrumental movements.
For the melody, three close notes then a leap of at least a 3rd is a place to start. After the leap repeat from there. Notes from the chord's pentatonic is also a good first draft method.
A string of notes is a string of notes, write in phrases. Let the story, and or melody breath.
You just wrote a lead sheet, now go back and put in the bass clef bass line.
Good luck, it will be a journey.