Who know at what temperature the viruses start to denaturate? and why when you use the autoclave, you always rise the temperature to 120 centigrades and no more? thanks.

It depends on the virus and of course, the length of the heat treatment. Literature values (known to me) vary widely between 45 to 100° for a few seconds up to an hour.

Most widely actually 121° C is used. Theoretically you can go higher and in some instances it actually makes sense. In most cases, however it is sufficient to ensure proper sterilization. This particular value has probably been established as a reference value for industrial/medical sterilization protocols.

Probably because it is a standardish value, and various equipment is designed to reach that temperature in particular, and likewise lots of equipment is designed to survive autoclaving at that temperature.

121C is used because it's the boiling point of water at 1 atmosphere gauge pressure. It's an arbitrary choice, but that's where the funny looking number comes from.