Although this isn't the place to ask a question related to "Naruto Base", I was unable to find any other place.
1. How is one supposed to learn how to enter Sage Mode?
2. How do "Battles" between different people's biography's end?
3. How does one master different elements?
4. To attack another village, does one simply need to say "I am calling out the Kage, come and fight me"?
5. Are there the names of the Kage mentioned anywhere (Kage of NB)
1. You mean RP? You need to have met requirements such as having a certain rank (based on post count) and be granted permission through winning RP events.
2. Each rank has health and chakra amounts. You'd expect the match to be over when one person reaches zero but I've never seen that actually followed. Most people battle until you can force your opponent to take a hit, that is they can't counter your attack or somehow avoid it. There will be people who can explain this much better than I can.
3. There are people on NB who are given permission to teach RP. You can contact these people and they will explain the different elements and how you use them in RP. Be warned that a lot of people do request RP training and there isn't an abundance of people able to teach, so you may be put on a waiting list for a while.
4. I don't understand RP wars between the villages but I would imagine you'd have to be part of a village before you can participate. You may want to ask someone who has a village bar above their avatar as they are in a RP village.
5. I'm unaware of any list but I've never really searched for one.
I've never RPed so I'd encourage anyone with experience to point out any inaccuracies.