Is my text lacking of citations? I've started my Masters in Computer Science this year and i'm facing some questions about citations. Papers and dissertations i've seen so far have a lot of citations even for the most basic subjects. 

Now, i'm working on a paper and i have this question: should i explain things with my own words or use anyone's definition and properly cite their work? 

I'd like to know what is the usual way of doing citations. I don't know if i write with my own words and look for citations (or even when to cite). 

Thanks in advance.
To avoid plagiarism you need to cite most things that you use (exceptions below). It isn't a question of paraphrasing or not. If you use it cite it. Sometimes it is better to quote than paraphrase, but using other people's actual words is more a question of copyright than plagiarism. Plagiarism is making it seem like the work of others is actually your own. 

The main exception is "common knowledge". You don't need to cite the fact that the Earth is roughly spherical and not flat (Sorry, Terry Pratchett, RIP). You don't need to cite the definition of the derivative in math. But if you get something out of a paper, it is most likely that it needs citation. 

And, citation requires more than just the name of the original source. A reader of your work needs to be able to go back to the original and see the complete context of the idea, including the paper it came from and any references that appear there. A chain of evidence is vital to scholarly work.