In mot we start with two orbitals.We get two mo in return in which one is bonding and one is antibonding that have opposite wave functions sign . If s orbital has a uniform sign of wave function and we start with two s orbital of same signthen also we obtain two mo of opposite sign. How is this possible?

The  approach provides an extremely simple method for determining the electronic structure of many molecules. It is a bit simplistic, however, and does have trouble predicting structures for a few molecules. Nevertheless, it gives a reasonable structure for many molecules and its simplicity to use makes it a very useful tool for chemists.