Since the park is quite isolated, the best option is to get an own/rented car. There might be tour buses, but I prefer to explore on my own.
This gives you the possibilty to move freely around the park. You really must as the heat and the size makes it almost impossible to cover it all on foot.
To get to the park, you must head north on the I-15 from Las Vegas, drive for about 55 miles until the Valley of Fire exit.
Some roads are not paved, so drive carefully../
Valley of Fire State Park is only 55 miles from Las Vegas so you can certainly do it as a day trip but don't short change this great little park. It's not that you need a lot of time to explore but the best times to do it are early morning and evening. If you do a tour, you'll show up midday when the only heat you will experience is the temperature. You'll miss the true colors of this spectacle of fire. We arrived from the North Rim of the Grand Canyon which was a 5 hour 230 mile drive that took us from Arizona, up through southern Utah, and into Nevada. The east side of the park is a bit more remote as it is 20 miles further from Sin City. In fact, the entrance station was not even manned. The entrance fee is $5 per carload but that is rolled into the camping fee of $15 so one more good reason to camp.
We rented a car and drove out from Vegas. There are tours as well, but I would imagine exploring on your own would be a better time.
If you drive please stay on the roads, cars do terrible damage to the park. Damage that takes years to be repaired.
I included a link to a website with tour info
Once you've motored up the Interstate and made the turnoff (I found out where to turn from my guidebook) -- the road seems to go off into the desert blissfully unimpeded by signage. I headed out this way alone, in a strange rental car, with no cell phone, on a weekday morning in May. There was no one anywhere. I wondered "Am I going the right way?" I wondered "What will I do if the car breaks down?" I wondered "Where the hell does this road end up?" So I kept driving; the road is well-maintained and eventually you *will* reach the entrance to Valley of Fire.
There are tours, but if you have a car that's the best way to visit because you can choose your own pace and see what you want to in the order you want to.
To see how to get there by car please click on the map to enlarge it!