as mentioned above…talk with the Priest in confession…(and even if you could give to charity instead…since you are 14 and unable to pay it at once…I am sure it could be done as you can…like a small amount a month)
as to mortal sin…
you would have to judge this based on the moment you did the sin (and if one does commit a mortal sin…one would need to goto confession before Communion)
But in anycase…you will want to mention it in confession for the advice even if it was not a mortal sin.
From the Compendium of the Catechism:
- When does one commit a mortal sin?
One commits a mortal sin when there are simultaneously present: grave matter, full knowledge, and deliberate consent. This sin destroys charity in us, deprives us of sanctifying grace, and, if unrepented, leads us to the eternal death of hell. It can be forgiven in the ordinary way by means of the sacraments of Baptism and of Penance or Reconciliation.
- When does one commit a venial sin?
One commits a venial sin, which is essentially different from a mortal sin, when the matter involved is less serious or, even if it is grave, when full knowledge or complete consent are absent. Venial sin does not break the covenant with God but it weakens charity and manifests a disordered affection for created goods. It impedes the progress of a soul in the exercise of the virtues and in the practice of moral good. It merits temporal punishment which purifies.