The Arc de Triomphe is a must-see when you visit Paris. It's at the end of the Champs Elysees. You can walk to it from the Louvre... granted it's a long walk but it's doable. During the winter, there are gorgeous string lights on all the trees.
Before you go up the Arc, there are a few things you have to do. First visit the McDonald's. It's on your way to the Arc and it's gorgeous. It's got fancy paintings and statues as well as an assortment of pastries, fancy coffees, and delicious eats. Also you get clean bathrooms.
Second, follow the signs to go to the Arc. You have to go underground to get to the Arc. While underground, you'll see an area to buy tickets. If you're a student (18-25) you get a discounted ticket. Children up to 17 are free. Here's the site where you can get the tickets in advanced.
You'll also have to go through security before you embark on the journey to the top. Keep in mind that you will be going up a spiral staircase and there is no elevator. You will be tired by the time you get half way, but the key is to keep a good pace and to rest halfway in the middle. There is a place to sit and wait.
After you've rested, the rest is a breeze and the views are incredible. You can see the Eiffel Tower light up from the top beginning around 6/7 pm. As you can see from the video it's stunning. You're literally in the middle of Paris and you can have a 360 view of the city.
When you've soaked in your fill of views, moments with friends and loved ones, and the Parisian night air, make your way down the stairs.
At the bottom, if you're there in the evening around 6 in the evening, you'll hear people singing tributes to the tomb of the unknown soldier. You'll see flowers and a flame that never extinguishes and names inscribed on the walls. Not sure if this will happen to you, but it certainly happened to me. When I was there it was one of those moments that grounded me in the sense that I am just one part of this massive world that has so much history. It's a good place to check in with yourself.
Do yourself a favor and see it at night and in the day, the moods are totally different. Let me know what you think when you see it.