Well there's several ways. The most productive ways at level 20 goes like this.. Save up 13 strange coins and buy a piece of armour from xur. Rare items (blue) go up to a maximum of 21 light after being upgraded. So with 4 upgraded blue pieces of armor takes you to maybe 26-28. To get the blue items you need engrams or the actual items as drops. Best thing to do is the strike playlists. Higher the playlist the better rewards. Chance for rare, legendary and even exotics in the 26 strikes. By doing the strike playlist your also gaining vanguard rep and marks. So you'll eventually be able to buy yourself some legendary armor from the vanguard vendor. Then after getting to 28/29+ start doing the weekly heroics and nightfall strikes. At 28 I would start doing the vault of glass on normal. At 30 start doing the vault of glass on hard and then crota. Also completing the Eris quest line will land you with some legendary gauntlets. If your on ps4 I can help with most of these.