It's hard to believe you would be given an assignment to write a cantus firmus but not have a text book or syllabus for procedures. The whole point of species counterpoint is to learn about rigorous compositional procedures. How can you do that without the procedures? If you did get that, ask for it.
Here is an example syllabus for a counterpoint class and it gives a list of rules for the cantus firmus.
You can also get similar stuff from other counterpoint books. Look for Fux's Gradus that's the classic text. Goetschius' Applied Counterpoint has a detailed intro for melody rules, although it isn't specifically labeled cantus firmus.
Some of the rules I've seen in various sources:
- start and end on the tonic
- use a modest range like an octave or tenth (it should be easily singable, that's the real goal of the cantus)
- move by step, diatonically (except the penultimate note may be a raised leading tone like
D dorian ... and some others like a lowered subdominant in lydian, etc.)
- keep leaps small, generally the max is a perfect fifth (an octave is acceptable too)
- resolve leaps by step in opposite direction
- melodic contour-wise place the climax, highest note around middle or slightly after middle of the melody
- avoid chromatic, augmented, diminished, or tritone melodic intervals
- avoid repeating a note too many times, max of three
- avoid repeating figures or melodic sequence
- avoid outlining a triad (I think I have seen this somewhere, but can't quote it)
You cantus seems fine to me, but the opening outlines a triad, and mm. 7, 8 and 9, 10 are a melodic sequence.
Clef and key don't matter.
One final point: don't get too hung up on the "rules." The cantus firmi that Fux presents in the Gradus include example of consecutive leaps, leaps resolve with leaps, etc. Try to stick to the rules for the sake a rigorous study, but understand they aren't inviolable.