How / when does neurogenesis occur while learning? I've heard that learning to Juggle (new things) helps increase neurogenesis. So, what does it depend on? 


Does it have to be a physical skill only? 
Or can I study / read about Economics(which is new to me) for that?
Should I have new emotional experiences like death in family, first time love etc.?


Also, when we try memorize something, say a poem, how is it decided that this has to be stored in the existing neurons or in the new neurons.

I'm aware that while connecting the dots, new connections are formed.
It has been discovered that neurogenesis takes place only on the hippocampus. 

Quoting Leonard E. White:


  We now agree that the production of new 
  neurons in the human brain is really 
  limited to a special set of stem cells 
  that seem to sit in the basal region of 
  dentate gyrus of the hippocampus. 
  So in this subgranular zone, we have a 
  population of neural stem cells that can 
  differentiate into interneurons that are 
  found in the granule cell layer of the 
  hippocampus. 


Also as you said, this production could be modulated through some interventions (e.g. learning)


  This genesis of new 
  neurons in the hippocampus can be 
  modulated. 
  By all kinds of different interventions 
  including pharmacological agents such as 
  antidepressant medications. 
  As well as physical interventions, such 
  as exercise. 


Also, this has been mentioned in this ted talk:

https://www.ted.com/talks/sandrine_thuret_you_can_grow_new_brain_cells_here_s_how

In summary, the following activities impact positively on neurogenesis:


Learning
Sex
Running
Good diet


While these ones impact negatively: 


Stress
Bad sleep
Ageing


pd: Concerning how memory works, my knowledge there is limited.