If your thesis has to be written according to the APA rules, you can do the online tutorial Basics of APA style of the APA organization itself.
The APA rules demand that you add the DOI's (digital object identifiers) in your reference list. To find the DOI of an article that you cite in your thesis, you can use this DOI-search programme. Please note that you have to enter the exact title. If the search machine does not find the paper, this doesn't imply that the article has no DOI, and you should look elsewhere.
In the Master Thesis Lab you can check the official APA-Manual.