1. Keep saving up marks for legendary armor.
2. Assuming you don't have either chest or helmet from legendary, then go do a queen's bounty in the bounty list. Complete as many as you can, PvE ones are easier. Then turn them in. Go to orbit and there's an option for Queen's missions in the side bar on the left. Choose it, if you didn't get the earth mission return to orbit immediately. Rinse and repeat until you get earth each time. Complete the mission and you are guaranteed either legendary helmet, legendary chest, and sometimes a legendary ship or sniper (but helmet or chest is a guarantee). Then you'd have even one more piece of gear.
3. If you can buy an exotic piece of armor from xur. You need 13 strange coins. If once again it's a piece you don't have even better, put that on.
4. Once you are decked in as much legendary and exotic pieces you are good to go. Heck as long as you have legendary on level them up. Keep leveling them up up until before the vertical shield upgrades as those require ascendant shards. Those are better spent on raid exclusive gear and exotics. If you level up all your regular legendaries up until before the shards upgrades, then fully upgrade your exotic armor piece, you will be level 28. At 27 it's the recommended level to do the raid.