All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
First of all, we congratulate you for the blessing that Allaah has bestowed on you by embracing Islam. We ask Allaah to help us and you in being mindful of Him, being grateful to Him, and worshiping Him in the best manner and make Paradise our abode and yours. We advise you to seek beneficial knowledge, strive to perform good deeds and befriend righteous and pious women.
What you said to your grandmother, "I already went back to Church", is not ruled kufr just by uttering it. The scholars stated that the person whose Islam is proven with certainty, Islam is not removed from him except with similar certainty.
Al-Mullah ‘Ali Qaari said in Sharh ash-Shifa, “Our scholars stated, 'If there are ninety nine aspects that indicate that a Muslim has become a non-Muslim and one aspect indicating that he is still a Muslim, then the Mufti and the judge should act upon this one aspect; this is concluded from the saying of the Prophet 'Avoid applying the hadd (corporal punishment) on the Muslims as much as possible.''”
If a person takes the Shahaadah (testimony of fatith), then he becomes a Muslim, and this does not have to be done in public; please refer to fatwa 51648.
As regards ghusl (ritual bath), it is only recommended to perform it upon embracing Islam, and it is not an obligation, according to the view of the majority of the scholars, as was previously highlighted in fatwa 11140. Ghusl is done after embracing Islam, i.e. after taking the Shahaadah, because it is an act of worship that requires intention, and the non-Muslim is not eligible for it (intention).
Allaah knows best.