Welcome to the game. I am going to answer your question in a couple of ways.
Focusing on gear, there are a few categories. General loot can be good and have multiple bonus's associated with them. Like +2 sword with 2d6 Acid. Then there Crafted gear, that you will need to delve deeply into the game to do, but it's essential custom built gear. Lastly there is Named Gear, which is usually extra special, in some way. But even better, it will often be part of a set, which gives you even better bonuses.
So, you will typically be looking for named gear. However your access is limited if you are free to play, so let's talk quests.
There are a whole bunch of quests that are free to play.
There are also adventure packs. Most new adventure packs cost money. The older ones can be obtain in three ways. First, VIP's get access with membership, second you can buy them, and the third way is by playing the game enough to earn points, which you can then use to buy the adventure packs.
I mention the adventure packs, because most of the cool gear isn't free to play. If you are chasing gear, the path typically includes Keep on the Borderlands, Feywild, Redfens, Sands of M..., Raven Loft, and Masterminds of Sharn. And none of those are free to play.
But don't worry about that now. For now, go do the Korthos chain. Make sure you pair up the reward gear properly. See this page.
Korthos Island Sets -
https://ddowiki.com/page/Named_item_sets#Korthos_Island_sets
If you do all the quests, you will have a basic gear set. From then on just swap out gear, as it becomes available. In addition to loot chests, you can get gear from vendors around the Marketplace and in some of the Houses. There is also the Auction house. And that exists, because soon you will have TOO MUCH GEAR. You will be ignoring it in loot chests, because you don't have anyplace to put it. If you go through 10 to 20 levels of Free to Play, you will know if you like the game enough to invest some money. Then you can become a gear chasing addict like me.
Here is some general info I posted in another thread.
Have Fun!