I agree with the suggestion to check out the CubeMX software for clocking and pin planning. However, speaking as someone who rarely uses that, there is another resource you should know about. Every STM32 datasheet contains a full-page diagram called "Clock Tree." This shows you everything you need to know, on one page. When you're first getting started, the minutiae may seem overwhelming, but study it carefully and you can see, for example, the relation between SYSCLK and HCLK (HCLK is a prescaled---divided down---version of SYSCLK which runs the MCU core). You can also see the multiplexer that produces SYSCLK, which will show you what the SYSCLK options are: HSE, HSI, a PLL output, etc.