start a the kibab trail head and take the trail to phantom ranch the halfway point. from here continue on to the nort rim or turn around and head back to the south rim.There are other trails that are easier but the kibab is the quickest. The elevation at the south rim is at almost 8 thousand feet and at the north rim its almost 9 thousand feet. When i went it was in march and there was snow on the south rim and when i got to phantom ranch at 3500 feet it was 85 degrees. For the first time unless you are in awesome shape a rim to rim to rim run would not be advised go half way to phantom ranch and head back.bring water there are very few water stops.