Hello dear Hadi and other friends i want to clear the concept. First of you askes wher the energy comes from? The answer is very simple when atom approach each other or come in close proximity the enery of the system raises why because the positively charged protons and the negatively charged electrons offers some repulsion and energy of the system increases.
Secondly this energy help carbon to promote one of its electron to the empty Pz orbital now the three P orbitals and one S orbital combine to form four equivalent orbitals called SP3 hybridized orbitals in this way carbon forms four bonds. Finally the electron shared are always used by either of the atoms.