If you're absolutely stuck, here's a way to begin.
1. Choose a system of morality. (This is probably your criterion, if your case is set up simply; make your value "life" or "justice" or whatever the point of that moral system really is.)
2. Determine why and how system of morality permits killing an innocent person to save others' innocent lives. (Or, if you're negating, why it doesn't.)
3. Explain why, in 2-3 main points. Use lively--and, if possible, real-world--examples to give your arguments color.
And when it comes to the other side...
4. Look closely at your opponent's moral system. Here are some questions to consider. (Adapted from Brad Hooker's article, "Ross Style Pluralism and Rule Consequentialism.")
a. Is it internally coherent?
b. Does it match with what we know about human nature, through psychology, economics, biology, etc?
c. Is it generalizable to all people everywhere? (If not, why not?)
d. Does it provide a clear standard for action?
e. Is it warranted (justified)?
f. Is it intuitive?