1) Did you pre-heat the oven properly before putting the cake in to bake?
2) Did you beat the egg mixture enough?
3) A tip to get light cakes that dont't sink is to beat all the wet ingredients well, then gently fold in the dry ingredients and put into the oven asap. Never let your cake mixture stand for too long before putting into the oven.
4) If all else fails, try to make a cake from those box cake mix. Just follow the instructions. If it still fails, maybe your oven got problem?
hope it helps.