Let's go to the store (Uniqlo, Adidas, the grocery store, etc.) tomorrow.
Let's go to a store tomorrow. (Let's go shopping somewhere, but I don't know where.)
The difference between the two is that 'a' is the indefinite article, and 'the' is the definite article.
2つの違いは、「a」は不定冠詞であり、「the」は定冠詞です。
Use “the” when talking about something specific that all the speakers know about:
“Let’s go to the store tomorrow.”
Here it sounds like I want to go to one particular store and whoever I am talking to knows which store I am talking about.
Use a/an when talking about something general:
“Let’s go to a store tomorrow.”
This is general-it doesn’t really matter which store.
アメリカ合衆国 
