My goodness, your post brought back many memories of mine from nursing school, waaaay back in the day.
Okay, first: please try to take a deep breath. What I would suggest you do is: Ask your patient some questions. Do you have a history of high blood pressure? Do you remember what your last BP was?
Make sure you have selected the correct cuff for your patients arm.
If your patient denies a Hx of HTN, then maybe you could pump the BP cuff up to, say 14o mm Hg.
Have you tried asking your nursing instructor/professor for assistance/help? Contrary to popular belief/opinion, many nursing professors/instructors don't bite (well, maybe not in the beginning, smile)
Here is a website that might be of help to you. Please, I am in no way suggesting that your approach is the wrong way to take a BP. Just trying to help a fellow nurse
I guess what I am trying to say is....Some things take a bit more practice than others.
Let us know how it all works out for you.