Understanding oz and grams

Hello .how is everybody doing.
My name is chana and a few days ago i got the app. I am 5'2 and 115 st.
In November 2015 i have to attend my brothers wedding in Trinidad. If i can loose 1 or 2 st i would be happy so you will guess the stress level.
I am not following WW/Sw but trying to manage my daily intake.
I have a scale in grams but there are several things that i do not understand about using the app. I would appreciate some help and guidance pls.
Thank you