I wouldn't go as far as saying that "quad stacked" or "double stacked" pills don't exist, it's entirely possible for someone to put 200mg+ of MDMA into a pill, it's just extremely unlikely.
I don't think there's actually a way to describe what MDMA feels like, but Here's how I like to describe it: Think of every emotion/feeling you've ever had and put them all together in a giant pot. Now, take out every negative/bad one and erase it from existence. What you have left, is what MDMA feels like.
Like Bsiren and Cinder said, make sure to be around good friends, take breaks from physical activity, and stay hydrated, but I have a couple other things to point out as well.
First off, although water is good and the essence of life, don't drink too much of it! It is actually possible to drink too much water, and it's something a lot of people do on MDMA in hear of dehydration. Look up hyponatremia if you want to learn more about it.
And secondly, be careful of the people around you when you're on MDMA, especially your first time. It's pretty easy to be taken advantage of when you're on MDMA. Although you may think everyone's your friend and the entire world is awesome and great, the truth is it's not, and there are plenty of people out there (especially at raves and places like that) who are ready and willing to take advantage of your newly found, drug induced love for everyone. It's not likely this will happen, just be careful.
Lastly, try not to worry too much about what I (and other people) tell you to worry about. It is something you should think about, but keep it in the back of your mind. The whole point of taking MDMA is having fun, so try not to worry too much. Just be careful, and have fun!