articles on multiculturalsim in the United States
My favorite starting place for finding articles is the ProQuest Central database. You should see a link for it on the Find Articles page.
Once you have ProQuest Central open, you should see a search box, with check boxes for full text, and for peer reviewed. I suggest checkmarking both of them before doing a search.
I did a search like this: multiculturalism "united states"
and got many hundreds of full text articles. The next step might be to add additional words to the search in order to narrow down the results.
I hope that's enough to get you started!