Congratulations on your first Confession!!
Everybody has been giving some really great suggestions, but I didn’t see an answer to your core question…how do you know when you’ve completed your penance?
Usually assigned penances should be something concrete…three Hail Marys, or pray a family Rosary, etc. The advantage here is that there’s no question when you’ve done it! Yours is more vague. Your responsibility is to do what you were asked to the best of your ability.
So if your recollection is that you were to do it a ‘few times,’ then your penance is complete when you have done a few acts of kindness (I would say 3 or 4; maybe do 5 to be sure

). You don’t need to be overly concerned about doing it flawlessly…if the Priest intended you to do 5, and you do 4, you’ve done fine…as long as your *intent *was to do what you were instructed.
Having said that, you should keep doing good deeds anyway even after your penance is completed

which is probably part of what Father is trying to get across to you.
For the future, if you aren’t sure what Father means when he assigns a penance, don’t be afraid to ask! If he says, ‘do a few acts of kindness,’ you have every right to ask, ‘How many, Father? How will I know when I’ve completed the penance?’ It’s not supposed to be riddle!
Congratulations again on your first Confession. It’s a wonderful Sacrament! Make use of it regularly! God bless you.