Hi there makinit,
An activated yeast means (I presume minding the language barrier) a yeast that has been at least dehydrated, or at best a yeast that is already fermenting in a yeast starter.
Now making a yeast starter means to take half a liter of must, add about 100 grams of sugar and nutrient and add yeast. Let this start fermenting for about 24 hours, and when it is fermenting vigorously add it to the rest of the must.
So, if you have the time, do as above.
If you do not have the time just dehydrate the yeast as on package description, and add it directly to the must.
Luc