Unfortunately most of the advice I have to give you comes from having personally experienced things over the years. One of these is dehydration. I can’t stress enough how important it is to drink enough on the trail no matter the time of year or the weather. Of course, in hotter climates, it is especially important to bring enough water (and to consume it!) over the course of your hike.
My bad experience came on a pretty warm Summer day hiking in Pisgah Forest in North Carolina. I was hiking with two other people on a steep trail coming up from the valley below the Blue Ridge Parkway to the Parkway itself when I started to experience symptoms of what I later found out was dehydration: a general weakness and vomiting. I found myself stopping on the trail just to sit and rest and we finally did make it to the top but it was tough. Dehydration, if left untreated, can lead to heatstroke and death.
The Mayo Clinic suggests hydrating properly the day before you plan to do any strenuous exercise (I’ve posted a link to their site on this topic below). And you will want to bring lots of water on the trail with you and remember to drink it. I have gotten away from hydration bladders because I can’t tell how much I’ve been drinking. With a water bottle, you can just pull it out and see where you are. Do what is right for you. Obviously the bladder will hold more water and distribute the weight more evenly. Sometimes carrying Powerade or Gatorade is a good choice.
If you do start to feel sick or weak on a hike, and you are with a group, please be sure and tell your hike leader so the group can stop and give you some time to rest and hydrate. There is no shame in this and it can happen to anyone.