If I want to talk about things that are the same, which articles should I use?

Eg1: A school has the same uniforms (only size is different) for boy students

Do I say:

> 
> Daily, a boy student wears the shirt with the black jean...
> 
> 
> 

or

> 
> Daily, a boy student wears a shirt with a black jean...
> 
> 
> 

Eg2: A shop sells bags (all bags are the same)

If I come the shop, and want to buy only 1 bag, do I say:

> 
> I want to buy a bag
> 
> 
> 

or

> 
> I want to buy the bag
> 
> 
> 

If I want to buy 2+ bags, do I say:

> 
> I want to buy bags
> 
> 
> 

or

> 
> I want to buy the bags
> 
> 
> 

or no article?