I have a few tips. First is, ignore sheet DPS. By that I mean if you are comparing two items and one shows it does more damage than the other, that doesn't necessarily mean the item with more DPS is better. What you need to do is look at all the affixes that come with it. This game is all about experimenting with different items to see how they work.
Another tip is elements. You can use the crystal calcite to change the element of your gear. Once you start finding legend(red) gear you may want to use calcite to change all the elements to the same for max damage.
Like griffin said, there's no need to focus on end game gear yet, it will probably frustrate you more than anything. Keep grinding above floor 100 on the highest difficulty you can do with speed and no deaths. Try to equip your character in all legend gear and watch for the affixes and sets on these items. Sets are the green affixes and when you combine the right ones, it makes for powerful builds.
Check the forums regularly, check the build help sections. After a few weeks you will start understanding how certain affixes work with each other. Once you have played a while and have bunch of crystals and mythstones saved up, then you can start crafting. To be honest I barely know anything about crafting. I look at builds on the forums that appeal to me and modify them to my play style.
You can modify legendary gear with crystals to change affixes but I don't recommend doing this early on. You are going to need tons of these for crafting end game gear.
Also check out this thread: