How you get keto form from enol form?

This will only occur if the alpha carbon to the carbonyl group can be protonated. The keto form of acetone (for example), would be a -CH2COCH3. The alpha carbon has lost a hydrogen and is negatively charged, C2 is double bonded to the oxygen. Keto-enol tautomerization occurs to provide some resonance stablization by creating the enol form:

CH2 = C(O-)CH3. The double bond forms between C1 and C2, which pushes the electrons to the oxygen.