4. Collect Motes of Light and Planetary Materials
Don't waste your Motes of Light or any of your existing planetary materials like Spinmetal and Relic Iron on current Exotics. You'll want to keep a surplus of them to upgrade Exotics in The Taken King, which still require these ingredients.
If you played Destiny way back in the day and are just coming back now, you'll notice an absence of things like Ascendant materials, which are no longer in use. Don't worry about those.