Stick to the "three source" rule of thumb. Always use at least three sources for your research.
It's okay to use common phrases in your writing. Teachers understand that you are learning new vocabulary and expressions and that you want to use what you've learned in your reading and research. Improving your note-taking skills is the key to avoiding accidental plagiarism.
Here are some tips to good note-taking that will help you avoid plagiarism:
Practise Note Taking
You can use pages from EnglishClub.com to practise taking notes. Here are some example notes taken from the reading The Disappearing Honey Bee:
My source: "The Disappearing Honey Bee" | EnglishClub.com | May 10, 2009
If you want to use a direct quote, write it down exactly as it appears. In your notes write down the information you need so that you can reference it properly:
"One third of America's food supply is pollinated by the honey bee." http://www.englishclub.com/environment/honeybee-reading.htm
Continue note taking: Now follow the rule of thumb for sources. Research CCD and the decline of the honeybee from at least two other places. After you take more notes, write a paragraph or essay on the topic in your own words. Practise using at least three direct quotes. When you are done, write out your reference page.