A rak'ah in salat starts with

A rak'ah in salat starts with Takbiratul Ihram. Takbiratul Ihram is the opening takbir (saying "Allahu Akbar") that marks the beginning of the prayer. It is the first act performed in each rak'ah of the prayer.
When we begin our prayer, we raise our hands up to our ears and say "Allahu Akbar" to enter into the state of prayer. This is called Takbiratul Ihram. It signifies the transition from our daily activities to focusing our minds and hearts on worshipping Allah.
By saying "Allahu Akbar" and raising our hands, we are expressing our belief in the greatness of Allah and our intention to dedicate this time purely for His worship.
After saying the Takbiratul Ihram, we proceed to the next actions in the salat, such as Qiyam bi Itidal (standing upright), Sujud (prostration), and Ruku (bowing). But it is the Takbiratul Ihram that marks the beginning of the rak'ah.
So, in summary, a rak'ah in salat starts with the Takbiratul Ihram, which is the opening takbir that signifies the beginning of the prayer and our transition into the state of worship.