How do you use a dichotomous key to discover the scientific name of an organism

You start by answering the first question on the list which tells you what to do from there. Pretend your animal was a bird, you might have a key like this:

Does this animal have wings?
yes                   go to question 2
no                    go to question 6

Then you just continue until the list doesn't say to do anything else. i.e.:

Is the animal feathered?
yes                  bird
no                    go to question 4