Frog breathing

This is a tricky one to answer Ali as it varies so much. Some ampibians spend all their life underwater and so breathe (i.e. get their oxygen and get rid of carbon dioxide) through their skin. Those that do this usually have large gills (like the axolotol) or extra folds of skin (like the hellbender) to increase the surface area of the skin and thus get more oxygen. The male hairy frog even has lots of filaments of skin to help it do this. It makes it look like it has fur, hence the name.
Other desert frogs and toads would soon dry up if they kept their skin moist like most other amphibians and so breathe only through thir lungs (like we do) when they are not in water.
As for say, a 'normal' frog on land (like the European frog)  this varies considerably. Frogs use the lungs primarily to take in oxygen and the skin to get rid of carbon dioxide, but the amout that each is used varies with humidity and temperature. Breathing through the lungs seems to account for about 80% of the oxygen uptake and 30% of carbon dioxide loss under 'normal' circumstances, with the skin therefore accounting for the reverse (20% oxygen and 70% carbon dioxide).