Do you have any reasons why you didnt perform as well as you needed? nedical or personal i.e bereavement.
You can be enthusiastic and clear why you are motivated to go there
If you are speaking to someone who can decide on admissions, then you might impress them with your interest in the subject , plus that course in particular as well as what a positive student you would be.
You cna also tell them you are willing to be flexible about some of the modules or take extra options of you displayed a particular weakness.
Unil you know whats on the table, then you dont know what your negotiating position is. There may or may not be a spot. If there is no spot do tell them you wish to go on the waiting list just in case someone drops out. Hint about reapplying next year.
If you are nervouse then writ it down so you have the information at hand and practice what you are going to say. Its like an interview until you are informed otherwise.