"She likes coffee."
Translation:Elle aime bien le café.
There is a longer explanation, but (I think) a good rule of thumb is to use "du" if you can insert a "some" before the noun, and it will not change the meaning of the phrase
He is eating bread == He is eating some bread == "Il mange du pain"
In this sentence, adding some would change the meaning of the phrase
She likes coffee =/= She likes some coffee
The first one means she unconditionally likes any type of coffee, the second means she is picky about her coffee.
When you talk about a noun in general to mean an all encompassing idea, you use the definite article in French. "Dogs are nice" = "Les chiens sont gentils" because I am talking about all dogs in existence.