The quick answer is that you should always tell the reader where the information is from when paraphrasing what someone has written or said. However, there is no black and white answer that goes for every occasion and all subjects.
Common knowledge that won't be refuted usually doesn't need a reference. This can for example be dates for well-known events or other commonly known facts such as the fact that Swedish is the official language in Sweden. Facts you can expect that most people know without looking in an encyclopedia.
The fine line of what is common knowledge and what is specialist knowledge within different subject fields, and therefore requires a reference, is sometimes difficult to distinguish. If you are uncertain, it is a good idea to check with your teacher. However, the general idea is that all information that you have taken from another source requires a reference.
You should always have the reader as your frame of reference. It should be easy for the reader to understand what part of your text is connected to the different sources and what possibly is your own comments or conclusions.
Strive to make the text as readable as pooible and thus inserting the reference smoothly. The reader should no have to stop the reading in order to read out your reference. Also, there is no need to repeat the reference unnecessarily as long as it is clear that it still is the same source. If you have a longer paragraph that comes from the same source, it can be clarifying if you make the reader aware of the fact that it is still the same source that is being referenced. This is also a good way to, if needed, clarify that it not your own opinion that is presented. You do this by using reporting verbs together with the author:
There are a lot of different reporting verbs to choose from. Vary the verbs and choose according to what is fitting in the context.
If the paraphrase continues into a new paragraph, reintroduce the citation.
Read more about referencing on APA Style: Appropriate Level of Citation