Which one is correct from a native speaker's perspective?
"if you follow this line of behavior you will be able to achieve tangible results in everything you try to undertake in the nearest future"

OR

"if you follow this line of behavior you will be able to achieve tangible results in everything you will try to undertake in the nearest future"

Question

Which one is correct from a native speaker's perspective?
"if you follow this line of behavior you will be able to achieve tangible results in everything you try to undertake in the nearest future"

OR

"if you follow this line of behavior you will be able to achieve tangible results in everything you will try to undertake in the nearest future"

Answer

- you don't need the extra 'will' as you are already talking about the future ("you will be able to")
- "try to undertake" is strange, as to "undertake" is to commit to doing something - you can't try to undertake, you either do or don't undertake.
- "near future", not "nearest"

so:

"if you follow this line of behavior you will be able to achieve tangible results in everything you undertake in the near future"

Answer

Yes - I agree with @topo morto.

"If you follow this line of behavior you will be able to achieve tangible results in everything you undertake in the near future."

Capital 'I' and full stop (period) at the end.