The Orokin Reactor is an item used to supercharge Warframes, Archwings and Companions, doubling their mod capacity. If you do not plan to spend money on the game to progress (which you do not have to) opening the planets is the fastest way to level up and start the collection of mods, resources and parts to craft more items. That way you have access to all Alert Missions.

Probably the most important item to note is when you start you should try to open all planets and junctions. When you start if you are going to use your platinum, many believe the best use of that is to buy Warframe and Weapon slots.