Mobs of all levels drop items at the same rate.
I reccomend you to start grindind bounties and marks, because that will guarantee you access to high level gear.
When you up your vanguard level to lvl2 you will gain access to vanguard armors, and when you get to lvl3 you can purchase weapons. All purchases are done via Marks. You can get them via Public events [4], daily challenge missions [5], strike playlist [2-8, depending on dif], and by selling materials. 50 of any material will give you 4 or 5 marks. For each level of chriptarch [after 4th i guess] and vanguard [after 3rd] you will get postmaster message that will give you at least one legendary item/engram. For each 1st public event with gold rating per day, you will also get reward with postmaster.
If you play weekly challeneges, you will get Strange Coins, you can use them to buy exotic gear from strange vendor [13coins for armor piece, 20+ for weapons].
If you play at least a little each day, you will be able to solve bounties and earn few marks, easily filling up weekly quota [100 marks], which will guarantee you some good gear. If you have all purple armor pieces, you can easily upgrade them to be lvl27.
I will describe best grinding places later.