Mtashed has a superb 12 minute video posted on the intricacies of getting the best armour set for your characters. If you are struggling as a new player or perhaps just need some clarification as a veteran of Destiny 2 then you will appreciate this video. 

Here is what's covered:

The Base Stats

Obviously, some stats are more relevant than orthers. For instance Recovery is almost a better choice than Resilience.   Strength, Discipline and Intellect are stats that you should customize based on your playstyle. 

Boosting Stats

You can boost stats by simply putting in new pieces of gear or.... you can masterwork your gear to boost its stats.  If you masterwork all five pieces of a set you boost everything one level. This is min-maxing for the players who absolutely love the numbers. 

Mod Slots vs Total Stats

• In PvP the general rule is to take the items with the highest stat roll (the "Total" at the bottom of the stats list in the item details)
• In PvE you sometimes want to keep the item with the better mod slots. 

How Many Sets Should You Have?

You should have a Solar, an Arc and a Void set.   

"One of each set with an Undying Mod Slot" - Mtashed"

General Rule of Thumb

For PvP go with the highest total stats.   For PvE you need stats, but you also need maximum mod slots.    

Where Do I Farm My Armour Sets?

You can farm anywhere, but...raid items, all raids including Nightfall can give you high stats items.   If you want to get the best stats rolls possible do the raids. Hawthorn in the Tower will give you raid challenges that further increase the level.   Beautiful. 

You can farm the Nightfall and as part of your Season Pass  (Level 44) you get Strike Armor Scrounger, which grants a chance to receive an additional Season Pass Legendary armor drop from completeing strikes.