Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
According to the consensus of Muslim scholars beginning with the greeting by saying al-Salam Alaikum is a desired Sunna while responding the greeting is an obligation for the verse: {When you are greeted with a greeting, greet in return with what is better than it, or (at least) return it equally..} (4:86)
The other expressions for greeting cannot replace the Islamic greeting because when Umair greeted the Prophet (Sallallahu Alaihi wa Sallam) using the expression of pre-Islamic period "good morning" the Prophet (Sallallahu Alaihi wa Sallam) objected and said: "Allah honoured us with the greeting better than the yours, O Umair! That is al-Salam Alaikum the greeting of inhabitants of the people of the Paradise" ( Seerat Ibn Hisham ]
However, using the word "Hello" is permissible if it is used only for to get some one's attention. Above all, it became a global expression and wide-spread among the public. In spite of its permissibility, it is better to use the words of Islamic greeting.
Finally, know that using "Hello" instead of al-Salam Alaikum, or being limited only to it, is not permissible.
Allah knows best