Q: What citation style should I use to avoid unintentional plagiarism?

Majority of journals require APA style formatting of reference citations. The APA style guide provides a tutorial that will help you to learn how to use this style.

http://www.apastyle.org/learn/tutorials/basics-tutorial.aspx

However, the formatting of references according to APA style does not guarantee that your manuscript will be free from plagiarism-related issues. Plagiarism in the manuscript, whether done consciously or unconsciously, should be addressed by paraphrasing sentences and by providing the reference to the source article. Authors of a manuscript should ensure that they do not copy-paste sentences verbatim from a published source. 

So ideally, what you should do now is run a plagiarism check on your manuscript. You can use online plagiarism checking tools such as Plagscan, Viper, iThenticate, etc. for this. Once the software identifies the sentences which contain plagiarized content, you should rewrite these sentences using your own words. If there are a few sentences that you cannot paraphrase, you should put them within quotation marks. However, quotes should be used sparingly, and only when it is absolutely impossible to parapharase.

Recommended reading:

• Plagiarism in Academic Publishing
• 5 easy tips to avoid accidental plagiarism
• How to paraphrase English text effectively
• Is it plagiarism if I use the same words as the original text but cite the source?
• The complete guide to writing a brilliant research paper