why first electron gain enthalpy is -ve ans second electron gain enthalpy is +ve explain.

Please note that electron gain enthalpy will not be negative for all the atoms. 

Electron Gain Enthalpy or Electron Affinity is the amount of energy released when an isolated, neutral, gaseous atom takes up an extra electron to form uninegative gaseous ions.

X(g) + e⁻ ----→ X⁻ 

Negative value of electron gain enthalpy denotes that energy is given out and a positive value denotes that energy is absorbed when an electron is gained by an atom. More electronegative is the element, more is the energy given out when the atom accepts an electron. As metals have a tendency to lose electrons while non-metals have a tendency to gain electrons, therefore electron gain enthalpy will be positive for metals and negative for non-metals. However, after an atom gains an electron, it contains an excess of electron and therefore becomes negatively charged. As a result, the magnitude of electron-electron repulsion increase and hence, this ion formed will not gain another electron so readily. Secondly, the attraction of the nucleus for the incoming electron will be less. Hence, energy will be absorbed when this ion will accept second electron and therefore second electron gain enthalpy is positive.