@onarum: Normal enemies only drop mods (don't underestimate mods though, you'll end up wishing you got more of these). For weapons and Warframes, you'll want to find bosses.
The first boss you'll come across is likely to be Captain Vor on Mercury, he drops the blueprints for the Chronus sword and Seer pistol. Typically, each planet has a boss, which drops a number of blueprints and/or components. The second boss you'll come drops the components to a very tanky Warframe (Rhino, which a lot of players favor).
Worth noting is also that there's a concept known as "Supercharging", which doubles the amount of "power" per level that is available for you to power mods with (so you'd have 2 power at Rank 1, 10 power at Rank 5, 60 power at rank 30). The Orokin Catalysts (for weapons) and Orokin Reactors (for Warframes) are very rare however.
@ajamafalous: Fair enough. But that +3 damage vs -4 spread is part of the mod system. In many ways, one could draw similarities between Warframe's mod system how World of Warcraft's reforging system used to work, letting you decide which statistics to focus on. But maybe I'm misreading you on how Destiny handles loot, so never mind :)