[How do I buy more Pokeballs???]

Help!!!! I am out of Pokeballs and I can't buy more  
 

 

[how do I catch Pokemon?]

I can't catch pokemon I selected the ultra ball but it did not work how do I do it Help!!!!!!!!!!!!!!!!! 

Note: I am new (as you can see by my rank) and idk how to catch pokemon that made me sad so can you help me pls?