If you are between level 18 to 20, we have a great farming location for you in Destiny where you can find a lot of legendaries and rares. Now, why this works is because there are lot of enemies that spawn in this location so you can get good chances of finding great gear.
Destiny is all about gear and that determines how you can handle hard enemies that you will encounter later on in the game. This location is in Venus, so make sure you have unlocked it.
Check out the video below which shows the farming guide along with some commentary. Let us know if this worked for you.