There are a few ways to farm Endo in Warframe… these are the basic beginner steps which everyone can do :).
1: You will get a weekly, where you can do a challenge for a reward of an Ayatan Sculpture.
2: Go to Maroo’s Bazaar.
3: Talk to her and pick the last option. This will queue you for an instance. Run to the end of the instance to do her challenge. You can kill mobs, but you can also avoid them if you like, as the challenge may initially take you a few runs to complete, so its just better to get to the end to try completing it. When you are comfortable at doing her missions, feel free to clear the instance as well if you want.
This is a video on how to complete them if you are stuck. Though some of the fun is learning, as you can queue for them indefinitly until you complete one per week.
4: She will reward you with an Ayatan Sculpture at the end of the timed trial. When you are done, go back to Maroo. Press, ESC > Equipment > Mods, then click on > Ayatan Treasures.
6: Click on your Sculpture (list of Sculptures and their worth you can acquire can be found here, of course all are nice to get as its free endo). Then click on every circle slot in it, and place in a star until the object moves freely.
Tips for Stars:
To find more stars… you can Mod your warframe or companion pet to be able to help you find more.
7: Go back to Maroo and click Extract Endo. The reason you place in stars, is because a Sculpture by itself may be worth like 300 endo, and a star about 50, but if you put in 3 or so stars into a Sculpture, combined its worth about 1500-3000 endo because you have “completed” the Sculpture. Making it worth more. You can also place these in your ship for decoration… (or to keep for when you get poor later on :-)).
1: Go to Mod machine.
2: Click on the far right, Sort by: Duplicates.
3: Click on any mod that has a high number on the top left. That you know what it does and think, “i dont need 100 of these” kind of thing. Example, something like redirection or vitality drops a ton, players can have like 500+ of them at times, so it is unneccessary to keep them all.
Top left is the amount you have (in this example, 23).
4: You can check to see if you have any already leveled up (before selling all of them). Or you can just sell “most”. Example from above picture, sell 22 each time rather than 23, so you always have 1 spare (this can also be useful for robots guns, as they cannot share a mod with other guns you have equipped).
5: Click on the mod, and select the amount you wish to sell.
6: Then dissolve into Endo.
Types of mods will also change how much endo they give… (example prior was a bronze mod, example following is a gold mod)