If you have enough information (as is found in a bibliographic citation) to identify an article, look up the journal by title in the Library Catalogue. Be sure you look it up by the title of the journal, not the title of the article.
The catalogue entry might give you one or more links to the online version of the journal, or it might give you a call number for the print journal.
If the library doesn't have the journal, or doesn't have the issue that you need, you can order the article through Interlibrary Loan.
You can search for articles on a topic by searching bibliographic databases.
Bibliographic databases fall into two categories:
NASA Technical Reports Server (at http://ntrs.nasa.gov/?method=aboutntrs; not in the U of R Library database list)—Publications from NASA, including lots of physics, geophysics, planetary physics, and astrophysics publications. Many documents are available freely online; others must be ordered through Interlibrary Loan. More information about NASA's publication program is available at http://www.sti.nasa.gov/.
US Dept. of Energy PAGES (Public Access Gateway for Energy & Science)(at http://www.osti.gov/pages/; not in the U of R Library database list)—Still in beta mode. "When DOE PAGES moves beyond the 'beta' period, it will offer distributed full-text access to all DOE-affiliated accepted manuscripts or articles after an administrative interval of 12 months." At the moment, PAGES contains a sampling of articles reporting on DOE-funded research.