Here's the advice I gave my friend who is dayhiking Mt Whitney for her first time up it.
You'll get a wag Bag, but may not need it. Wag bag is easy. You shouldn't need it for a one day. Just try to use the bathroom at 2:30 AM. It's tough.... but. I have never used the Wag Bag on a day hike.... so really, no worries. If you do use it... Carry it out and dispose of it at the Portal when you get back.
You're gonna need headlamps. A first timer hike in one day is appx 16 hours + / - ...... Trail Camp should take you 6 hours appx. Drink a crap load of VitaLyte or Gatorade at the start before you even leave the campground. Bears will be walking around, so keep an eye on your gear. They won't bother you, but they do look for openings to take your backpacks.
You'll be using a day pack. Take it slow at the beginning. It's a long, long ways to Lone Pine lake. Again....slow. When you hit the meadow at Big Horn Park (again, hard to see in the dark, but there will be a stream to the right of you) you may be able to find the spring that comes in from the left. It's dark though, so you may not see it. It's a good place for water. You don't need to filter that water as it's straight out of the ground.
Stay quiet as you pass the campers at Outpost Camp. It won't turn light for a while, probably above Mirror Lake.... It's a bit tough to find the trail after Mirror lake, so you should keep an eye on hikers ahead of you to find the trail. Pretty self explanatory after that. You can get more water at Trailside meadow. Take a break there and rest a bit. Then up to Trail Camp, where you should make sure your water containers, bladders are full. Then another short break.
Sun should be coming out and hitting the mountains ahead of you. You may be able to see the 97 switchbacks. Make sure the Marmots don't take your food/ candy, etc. Then head up. Take your time. The 97 switchbacks take out the most people. That's why you need to take your time. Don't look up or look for Whitney, as it will just make it look longer. At the cables, there will probably be some ice. Just take your time and keep an eye on where you step. The cables are a great place because it means you are doing well and you should be able to make Trail Crest. Just keep moving, drink fluids, take Ibuprofen, Excedrin, whatever when and if you need it.
STAY with the slowest person and encourage them. Stay together as a group. You will then have one really long switchback at the end, the 97th. You will be at Trail crest. At that point, you will get a nice beak, take pictures and feel some energy. It's still a couple of miles to the summit. Again, take your time and watch your step. You will notice that you are going down for a bit. That means that you will have to do some going up on the way back after the summit, so conserve your energy and remember. It's pretty much in your head at this point. You just need to keep your legs moving, talk, encourage and stay hydrated. EAT if you can. One more large turn away from the summit, then up to the summit hut. You've made it!!! Sign the register, take pics, eat some food and hydrate. Don't stay too long as you are now at 14,507 feet and it's your first time. It can get to your head.
Now, the hard part.... the way down. Take it slow, especially from the summit to trail Camp. Those small rocks can now trip you up and you can fall, turn an ankle, or worse. So Pay Attention. Get back to the Portal and get yourself a Huge Burger and a beer... :-)
I would appreciate anybody else's advice for first timers....