So your new to Warframe, You have your starter frame and have started running the solar system map missions. Your unlocking junctions, making weapons, and getting new warframes. You may also have started doing alerts and have noticed ones for something called Nitain Extract. These you definitely want to do if your on and they are available . Why? Because as you progress through the game you will need 158 if you want to build everything that uses them. This is one of the resources you will probably never stop needing as the Devs keep adding frames, weapons , and cosmetics that use it and even with them adding it at an abysmal percentage to reactor sabotage Alerts are still the best way to get it.(for more information on what it makes check the wiki and if you need help getting to a mission feel free to ask clan-mates in discord.
There are a couple other things to look out for in alerts. Aura mods like steel charge, energy siphon and corosive projection, as they only come from alerts. Vauban parts as well only drop from alerts so make shure you run them whenever they pop up.
Worth mentioning that the ONLY way to get Nitain is by mission reward. You cannot farm it at all. So get it whenever you see it pop up as an alert mission even if you do not need it right away. You will need it sooner or later.
That is not correct simdor. Nitain can be farmed from lua exterminate missions, it drops from the hidden caches. Not garunteed but it is in the drop table. Though lua is a pain in the ass to find the caches, it can take a bit as the tiles are pretty big.
Oh cool I was not aware of that
@thorm Thanks for that info. I too did not know Nitain could drop from somewhere other than the Nitain missions. I currently am working on the Vauban Prime blueprint, and need 20 total for the build, of which I still need about 11 more. 
If you do look for the hidden caches on lua id recomend titania. she can fly around and get places a lot easier on lua.
I have Titania, so that is a great idea!