Recommended path:

Update11: The new update adjusted all enemy levels so that level 100 enemies are now level 40. However the exp gain has also been reduced meaning you will have to grind longer :( 
Kappa is still a really place to grind but you may also want to grind on apollodorus on Mercury (or any other survival or defense mission). 
It's a survival mission where enemies get progressively harder and you can access it early. However to get the most exp you need to stay as long as possible on one level and the problem with that is the longer you stay, the higher the chance you might disconnect meaning no exp. 

First I would focus on getting to Sedna and unlocking the mission on Kappa.

The reason I want you to unlock this mission is because it is a great place to level up your frames and weapons. 

• Kappa is a mobile defense mission and needs you to defend at three different locations. Stay near the object you are defending! I've seen quite a few missions fail because no one was paying attention to the objective.
• It has a lot of enemies who are around level 21, which is hopefully what you should be by the time you get there.
• Even if your gear is not near 20, higher levels also play this map in order to level up their weapons so you should have plenty of help.
• Also you can get a lot of mods here in order to upgrade. If you still have not got your serration or streamline mod then farming here will probably get you one.
• You could also level up a bit at Lith on Earth but those enemies are only near level 13 so it would take longer to max your weapons and frame.
• Just as a precaution when levelling. If you are going to level up new weapons or frames, keep at least one weapon at a high enough level to do damage or a frame that has its skills levelled up so that if your team is not doing well, you can switch weapons and take care of things. Note you still get exp if you fail the mission but you lose any mods and items you found.

The path:

Mercury – Venus – Earth – Saturn (top left from Earth after Everest) – Sedna (Do the missions leading to the top right from the start of Saturn map where Tethys is the last mission).

The Earth map. Go left after Everest.

Follow this path to Sedna.

Our goal.

Note that the boss of Venus drops Rhino blueprints.

You should farm him till you get the helm, systems and chassis.

Reasons for building Rhino first:

• Ability called Ironskin which when maxed can absorb 1200 damage! Also prevents you getting knocked down while active, making you a good tank.
• Rhino stomp does aoe damage and it causes surrounding enemies to be lifted in the air for a good amount of seconds when upgraded. If playing with a rhino and they use rhino stomp, lifting enemies, carry on shooting till their dead... I keep seeing people stop shooting for some reason.
• At the beginning your running speed will be the slowest of all the warframes, however once you get the alternate helmet(25% speed) from an alert and a nicely upgraded rush card you will be able to keep up easily with the rest. Alternatively build and equip dual zorens and do the jump slide spin move  mentioned earlier to travel.
• You will need a control module in order to build his and every other warframe's system. In order to get rhino early you should try asking for help in chat. Many higher levels won't mind inviting you to their squad and helping you get it.
• Whilst playing with higher levels be warned that some will just rush past enemies to kill the boss (due to boredom from repeating the same tileset over and over). Do your best to keep up and run past enemies without attacking, Loki's invisibility will help immensely. Not everyone likes rushing though (I don't) and will kill most things on the map. 
• If you cannot find a team then the best way to get control modules is by going to Europa or Neptune, preferably Europa because of the lower level monsters (30-42 as opposed to Neptune's 42-60). Hunting the boss is the best way to farm although you can get these in other missions but at a lower rate. You could also get them from alerts but they are pretty rare.

After you are done leveling up you may continue with the rest of the maps however after finishing mars do not go Phobos! I found those enemies too high for the weapons available if you went Venus – Earth – Mars – Phobos. Using my modded boltor at rank 30 it took a full clip for me to kill each enemy on that map and I have no idea why it's after mars :p, just ignore it for now.

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