What is the first part of the Calvin cycle?

It refers to the initial incorporation of #CO_2# into organic material. We will follow #3# molecules of #CO_2# through the reaction because we want sugars(carbohydrates) as the end product of cycle. And for this, at least #3# molecules of #CO_2# are necessary.

Because;
#3# molecules of #CO_2##to # contain #3# #C to# so at least they will give #1# carbohydrate molecule( a triose).

• Calvin cycle begins by the reaction of #CO_2# with highly reactive phosphorylated five carbon-sugar named ribulose bisphosphate(RuBP).
• This reaction is catalyzed by the enzyme ribulose bisphosphate carboxylase also known as Rubisco.
• The product of this reaction is highly unstable six-carbon intermediate that immediately breaks into two molecules of three-carbon-compound named #3#-phosphoglycerate(PGA).
• The carbon that was originally a part of #CO_2# molecule is now a part of an organic compound which means carbon has been fixed.
  

https://www.khanacademy.org/science/biology/photosynthesis-in-plants/the-calvin-cycle-reactions/a/calvin-cycle

#Note:#
Rubisco is the most abundant protein in the chloroplast and probably the most abundant protein on Earth.