A Pikachu in HGSS has a 5% chance of holding a Light Ball. This is true whether it is your first encounter or your hundredth encounter. So, if you're only taking into account your Pikachu encounters, the answer is 5%.
If you found a Light Ball Pikachu as your first encounter in Viridian Forest, the odds are of course lower. We'll go with maximum odds here: Pikachu has a 6% chance of appearing in the forest in the morning and daytime. All you need to do is multiply the 6% by the 5% (for the Light Ball chance) to get your answer.
0.06 * 0.05 =0.003
= 0.3%
So a first encounter Light Ball Pikachu would be a 0.3% chance.
