Whenever you summarise, paraphrase or synthesise you must restate someone else's ideas in your own words and must always acknowledge the original source by providing a reference. It's not enough to substitute synonyms for key words. You must change both the wording and structure of the original sentences / ideas - without changing the meaning of the original - if you wish to maintain the integrity of your work, and avoid plagiarism.
It is advisable to summarise, paraphrase and synthesise from your notes. The main purpose of your notes should be to rewrite key ideas from a source in your own words. Therefore, these notes will serve as a record of your own understanding and thoughts about what you're reading, in relation to your purpose or assignment directive. For more tips, see writing from notes, as well as our advice on note-taking and reading strategies.
When writing your assignments, aim to summarise rather than paraphrasing one sentence after another - the danger is that you end up replicating the same structure of the original, and miss the opportunity to show critical understanding of where the idea fits within the overall big picture. It is thus generally a good idea to first summarise the entire article with your assignment question in mind to test your understanding of the source's argument and significance before paraphrasing key points. This way, it is easier to convey the ideas you wish to paraphrase in your own words and ensure that when you do paraphrase, you are not taking specific ideas out of their broader context.
Providing a citation any time you draw on other people's ideas, theories, data or material, will clearly distinguish your ideas from others'. Indeed, if you don't provide a citation in your work, it will be assumed that what you have written is your own idea and / or common knowledge. Your opinions, experimental results, experiences or thoughts can be considered your own ideas. However, ideas don't occur in a vacuum and if it's inspired or influenced by others, you should always provide a reference, as referencing provides that important and expected acknowledgement.
Sources are wide and varied, and scholars from different disciplines prefer certain sources over others. Take a look at the section on Researching and finding sources for more details.