I assume that my inventory items should be used for selling for cash and/or constructing gear and weapons, however I don't see any way to do either of those things. I'm at Level 13, does this come into play later in the game?
Some of the materials are used to upgrade blue and higher gear. Some of them (the ones you collect from resource nodes on patrols) are used for upgrading purple or higher gear, but can also be traded in for crucible and vanguard rank and marks.
There are also some quest lines that use some of the various things, including consumables, for exotic gear (yellow level, above purple).
There are also a few different kinds of currency that show up in your inventory, mainly strange coins and motes of light. You'll use those later in the game to get high end gear.