Hello
First thing is your MCU selection. It is going to depend on the nature of your project
Automotive (STM8AF/AL)- NonAutomotive (STM8S/L)
Mainstream or Low power (STM8S or STM8L)
Then I suggest you to start with STM8CubeMX and select the appropriate P/N, and configure the device according to your needs.
https://www.st.com/content/st_com/en/products/microcontrollers-microprocessors/stm8-8-bit-mcus/stm8s-series/stm8s103-105.html#tools-software
Several IDE are available (depending on your preference), offering Free C compilers
https://www.st.com/content/st_com/en/products/microcontrollers-microprocessors/stm8-8-bit-mcus/stm8s-series/stm8s103-105.html#tools-software
https://www.st.com/en/microcontrollers-microprocessors/stm8s-series.html#tools-software
Ordering the 64pin Nucleo for the Hardware is a good start as it comes with different exemples. it is a quick hands-on solution.
https://www.st.com/en/microcontrollers-microprocessors/stm8s-series.html#tools-software
For programming the STlink-V3 will be useful for STM8 and STM32.
I hope this will help you to initiate your development.
Kind regards