First and foremost:
WARFRAME WIKI (Read it. All of it. Except the parts with spoiler warnings.)
If you can't choose what frames/weapons to use, use this. Read it carefully.
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How Warframe works:
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Try to unlock all the planets as soon as possible if you don't like spending platinum on potatoes (see #3) and cosmetics!
1. Do missions.
Warframe has 14 main mission types:
Assassination - kill a boss and unlock the next planet
Capture - kill an enemy who tries to run from you and steal his corpse
Deception - take an item to the enemy computer core and insert it
Defense - defend an objective against waves of enemies
Excavation - power and defend some drills
Exterminate - kill all the enemies in the mission
Hijack - protect a payload that's losing health and drains your shields to move
Interception - capture control points (warning: innovative, original game design)
Mobile Defense - take an item to 2 or 3 different computer cores and defend those for a few minutes each
Rescue - go into an enemy prison and rescue a hostage
Sabotage - blow up the enemy's stuff
Spy - hack into enemy computers inside data vaults and steal their dank memes
Survival - kill stuff, don't let life support reach 0%
*some mission types give more rewards after every 5 waves/1 drill/1 round/5 minutes and increase in difficulty
Special missions:
- Quests
- Alerts
- Invasions
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2. Get loot.
Loot includes:
Credits - the main currency in Warframe
Resources - used for crafting
Affinity - XP for frames/weapons
Mods - cards used to upgrade frames/weapons
You can pick up loot during a mission and you usually get some at the end of missions.
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3. Use loot.
Mods
Mods improve your frames and weapons. You can install them by selecting your frame/weapon in the arsenal and clicking upgrade. Frames and weapons have a mod capacity equal to their level. The capacity requirement of a mod is in the upper right corner.
Without mods, your frame and weapons are useless in almost all of the game's content!
Improve mod capacity with:
Orokin Catalysts(weapons)/Reactors(frames) - Doubles base capacity to 2 per level
Auras(frames)/Stances(melee) - Auras benefit your team and increase your Warframe mod capacity. Stances let you do combos and increase your melee mod capacity.
Matching mod polarity - Some mod slots have symbols in them. Match the symbols in the slots to the symbols on your mods to cut the mod's drain in half (or double the effect of Auras/Stances).
Forma - Craft Forma and use them to change the polarity of a mod slot on a rank 30 frame/weapon and reset it to rank 0.
Improve the effect of mods (but increase their drain) by fusing them at the mods section of your ship. Fusion works best with Fusion Cores (special mods that are for this) or duplicates.
Do not use damaged/cracked mods!
Crafting
Crafting can be done at the foundry in your ship. Crafting requires resources and credits.
You can buy blueprints for common items and weapons in the market using credits. Most Warframes can be crafted with part blueprints that drop from planet bosses and Warframe blueprints from the market.
If you're in a clan that has research done you can buy blueprints for more frames and weapons in the labs in the clan dojo.
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4. Platinum!
Costs real money!
Only buy cosmetic items, inventory slots, and Orokin Catalysts/Reactors from the market if you're spending platinum. Everything else is overpriced. Classic Saturated is the most versatile color pack. Smoke Colors contains a true black color, which can stop certain particle effects. You can buy a lot of holiday themed color packs without platinum. Use cosmetic items in your arsenal.
When you reach Mastery Rank 2, you can trade with other players for Mods, Keys, Prime parts, and platinum at a trading post in a dojo. Daily trades are limited by your mastery rank. You can not trade your starting platinum.
If you buy some platinum, try to get a Corrosive Projection aura from another player for about 20p. It reduces all enemy armor by 30% when fully fused and many players will let you join their missions if you have it equipped.
Check prices at warframe.market.
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Other stuff (still important)
Mastery Rank - Your overall level. Increasing it unlocks lots of stuff.
Orokin Void - Get shiny prime stuff here (but only if you have a key). Related: Orokin Derelict.
Sorties and Trials - If you can't look these up yourself, you're not good enough to do them.
PETS: Kubrows and Sentinels
The Carrier sentinel is the only one worth using if you like picking up loot in a reasonable amount of time. Carrier
Prime has more health and armor.
FAST XP: Draco(Ceres). Interception.
The goal here is to win as slowly as possible (capture C/D, neutralize A without capturing until you are winning) while killing as much as possible without using weapons.
This requires a specific build.
Squad includes an EV Trinity, RJ Excalibur, and a Buffer (Roar Rhino or FF Frost). Research these yourself.
If you have Corrosive Projection, you may be able to join anyway as a CP Leech.
FAST CREDITS/CORES/KEYS: Hieracon(Pluto). Dark Sector Excavation.
The wiki is your friend!
Warframe Subreddit /r/Warframe