Can a person who is still a Christian fast Ramadan, while believing in God and His Messenger and desiring Islam, but does not know how to fully embrace it?

Your fasting without embracing Islam is of no benefit, as acts of worship are not accepted except with correct belief and a sound religion. The most important step is to embrace Islam by uttering the two testimonies of faith: "I bear witness that there is no god but Allah, and I bear witness that Muhammad is the Messenger of Allah." After that, all other acts of worship can be performed. This feeling of desire for Islam is a good sign, so make the decisive choice for your salvation, for life without religion and values has no worth, and after death comes accountability, either to Paradise or Hell. Do not delay in this, for life passes by, and regret will be of no avail after it is too late, as stated in the verses: (Until, when death comes to one of them, he says, "My Lord, send me back that I may do righteousness in what I left behind." No! It is but a word he is saying; and behind them is a barrier until the Day they are resurrected.) [Al-Mu'minun/99-100], (If only you could see when they are made to stand before the Fire and say, "Oh, would that we could be returned [to life on earth] and not deny the signs of our Lord and be among the believers.") [Al-An'am/27], (And for those who disbelieved will be the fire of Hell. No end will be put to them, so they will die, nor will its torment be lightened for them. Thus do We recompense every ungrateful disbeliever. And they will cry out therein, "Our Lord, remove us; we will do righteousness - other than what we used to do." Did We not grant you life long enough for whoever would remember therein to remember, and did the warner come to you? So taste [the punishment], for there is no helper for the wrongdoers.) [Fatir/36-37].