http://www.jimmys-adventures.net/hiking-half-dome-tips/
Hiking up Half Dome in Yosemite, CALIFORNIA?
I need to know some tips >< Cause my Cousins and my bro hiked up there last 2 days ago at Yosemite and it took them 13 hours to get up there and down. Do you need to get in shape before hiking up there? How bad is it? Cause my cousin wanted me to do it like soon, but im scared i might fall. Cause one fall = you die.
Is this hike steep? Im just 13 and i need to know before doing this dangerous thing 
Average people hike up and down Half Dome every day. 10-12 hours is about the average time to get up and down. You do not need to be in excellent shape to make it up, but people who never exercise should think twice. You are 13, so you should be fine. Make sure you wear good shoes. I was fine in my running shoes.
It can be a little intimidating when you get to the base of the dome. There is a steep ascent with cables running up to the top – you will have to put on some gloves (provided at the base of the cables) and use your arms to pull yourself up. You only have to worry about falling (or dying) once you are on top. People have fallen off Half Dome and died, but keep in mind they were also doing something stupid (like showing off for their girlfriends by doing hand-stands, true story).
Go do it, and be proud of yourself. 
Hike Half Dome: Anyone Can*