Praise be to Allah.
You committed a sin by engaging in foreplay with your wife until you ejaculated, because by doing so you invalidated your fast and violated the sanctity of this blessed month, and you missed out on the reward of fasting, concerning which Allaah says: “He gives up his desire and his food for My sake.” (Narrated by al-Bukhaari, 1761).
Your regret for the action that you did is repentance. The Prophet (peace and blessings of Allaah be upon him) said: “Regret is repentance.” Narrated by Ahmad, 3558; Ibn Maajah, 4252; classed as saheeh by al-Albaani in Saheeh Ibn Maajah, 3429.
By ejaculating you broke your fast on the day when that happened, but you should refrain from eating and drinking for the rest of the day, until sunset. And you have to fast another day to make up for that day.
With regard to the kafaarah (expiation), if you had had intercourse with your wife, the expiation would have been severe, namely freeing a slave; if you could not do that then you would have to fast for two consecutive months; and if you could not do that then you would have to feed sixty poor persons. See question no. 22938
But if you ejaculated without having intercourse then you do not have to offer expiation, rather you only have to make up that day.
Al-Nawawi (may Allaah have mercy on him) said: “If he kisses his wife or has intimacy without penetration, or touches the hand of a woman with his hand etc, then he ejaculates, he has broken his fast. Otherwise (his fast is not broken).”
Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on him) said:
If (a man) touches his wife, whether he touches her with his hand, or with his face by kissing her, or with his private part (without having intercourse), then if he ejaculates he has broken his fast, but if he does not ejaculate then he does not break his fast thereby.
Al-Sharh al-Mumti’, 6/388
This ruling applies both to you and to your wife. If she reached climax as a result of this foreplay then she also broke her fast, and she has to repent to Allaah and make up this fast on another day. If she did not reach climax then she does not have to do anything.
And Allaah knows best.