The most XP you gain you will level up. The best place to start farming for XP is Earth, beyond Earth the missions are getting more and more harder. On Earth you will also encounter your first Defense Mission where you will actually have to defend something like a Power Core from waves of enemies. After Earth you will want to go to Kappa planet or you can search on the internet for the best defense planet that suites your desires. I can recommend you to go to Xini as this is a very popular map which a lot of players are using it for gaining Experience Points.
Warframe Farming For XP
Another popular spot for farming for XP is Cyath which will definitely increase your XP since it is a Mobile Defense. Some players prefer Kappa since it is a mobile defense and it will deliver more XP and it is relatively in beginning of the game, right after Earth. But even they reach further planets they still go for Kappa.
Best Places to Farm XP Video
These planets are also good for farming Materials as you will encounter a lot of mobs coming to destroy what you are defending, hence increased chances of getting more and more materials. This being said I hope you understand pretty much of it, it’s not very hard. Even so you can always use the comment section in order to ask your questions and I will happily respond to them. If you want to find out about farming for materials and warframe parts you can go >>HERE<< and read my tutorial.
Warframe Best Guides 