I want to spend xp to up grade a skill from 0 to 1. However, how do I know how that upgrade will modifiy the dice pool. I can't find it in the begginners rule book?

Using 5XP for a career skill or 10XP for a non-career skill to upgrade from rank 0 to rank 1 will permanently change an ability die (green) into a proficiency die (yellow).

So for example, if your Brawn characteristic is 3 and you're currently rank 0 in your Athletics skill your dice pool is three ability (green) dice.

Upgrading to rank 1 in Athletics skill will make your dice pool two ability dice and one proficiency die.

Compare the characteristic the skill falls under to the skill. Whichever number is highest is your total number of dice, whichever number is lower is the number of dice that get upgraded to proficiency dice.

So say your guy was kinda dumb but happened to be a savant with computers

2 Intelligence, 4 computer skill. He would get 4 dice total, 2 of them being yellow proficiency (the other 2 being of course regular green dice)

If the numbers happen to be the same, then ALL of the dice you roll will be proficiency dice.

3 cunning, 3 deceit = 3 dice, 3 upgraded to proficiency = all 3 yellow dice