The requirements for a valid Nikah are:
Mutual consent of the bride and the groom.
A guardian (wali). This is usually the bride's father, or her grandfather, an elder brother, or even an uncle. However, if there are no relatives, an appointed Muslim authority can act as the wali.
Witnesses - two adult, sane Muslim witnesses to the contract (two women can fill in for one man). Some mosques may require that the witnesses know at least the bride, or ideally also the groom.
The dower (mahr), a gift from the groom to the bride. This doesn't have to be exquisite - anything simple and affordable is sufficient.
The Imam's charge can vary depending on the mosque and the Imam. This is something you should best check with the mosque where you intend to get married.
The fact that your bride is a Christian does not change the requirements significantly. The principal difference lies in the requirement of a guardian. Some schools hold that a wali is necessary for all marriages, while others may hold that it is not required when a Muslim man marries a woman from the People of the Book.
Please note that this is a general answer based on Islamic principles. The requirements and their interpretations vary across regions and denominations of Islam, and may also be subject to state laws. Always consult local authorities for the final word on these matters.
