This is a really noob question, I know, but I need help with it.
The cooldown is a bit long and the damage done with the explosion isn't too much, so I need advices for a good use and cause I don't want to waste it nevermore.
Also this, those 12% damage increased will apply on the total damage of ignite?
I mean if ignite do 100 true damage, with my ult ignite will do 112 true damage?

As far as the ignite goes yes. Increases that damage also. And for when to use it- right after a team fight starts. Drop it as soon as they group up. If they don't group drop it on a carry to help turn the tide and burst them down

I try to land it on 3-5 people if possible,

But yeah, you want to use it earliest in a teamfight.