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.

Grind the strike playlists / daily / weekly for drops Buy from your factions around the tower!! Just play the game they will come !

Exotics, ranking up your vanguard/crucible rank, luck. I think the best way(if you're a lower level) is to grind out your vanguard rank. Once you hit rank 2 you can get armour. Once you have the rank, grind out on marks. Best way to do that is to do Vanguard ROC, or Vanguard Tigers. You get 6 each time and 25 rep. You can only get 100 a week, if you didn't know. I would recommend an exotic helmet, so it's easier for you to get to a higher level fast. To get commendations, once you hit rank 3 you get one and so on and so forth. You use the commendation for the chest piece. Let's say you have your exotic helm and all legendary armour. You upgrade to the part where you need shards. If you are new, I would say do the Vault of Glass on normal to get those shards! It is super easy to get the shards. For an exotic shard, you need to dismantle an exotic of buy one from Xur for 7 coins. I hope that answered your question, if you need clarification, just ask.