I have a MCU STM8(S105C4) with which I need to design a PCB to power from a USB. But I don't know how to power it. I've tried searching the specification for this MCU but all that I could find is that the input voltage should be from -0.3 to 6.5 V. The interesting thing it that for STM32 MCU's that the specification has a power scheme included. If anyone has any ideas that could help me out, that would be great.
The voltages you are referring to are the absolute maximum limits where the part will not be damaged if you apply said voltages.
What you want is the operating voltage, which in your case is 2.95 to 5.5 V as stated on the datasheet. This value is usually shown on the first page of the datasheet, and again a few pages further down, in section called Operating conditions, in your case page 54 of the datasheet.
It is then down to you to decide how to power the MCU, depending on what you connect to it, and your power source.