Google Scholar includes a "Cited by" count in its display of individual article entries. This is calculated from citations appearing in other publications indexed in Google Scholar. Clicking on this link will take you to a list of citing articles.
Google Scholar Citations allows you to create a personal profile to list your publications and calculate citation metrics.
Google Scholar Metrics uses h-index (as well a 5-year and median measures) to rank journals sorted in subject areas. Browse journal lists using the subject listing in the left column.
Search tips from Google Scholar:
Getting better answers
If you're new to the subject, it may be helpful to pick up the terminology from secondary sources. E.g., a Wikipedia article for "overweight" might suggest a Scholar search for "pediatric hyperalimentation".
If the search results are too specific for your needs, check out what they're citing in their "References" sections. Referenced works are often more general in nature.
Similarly, if the search results are too basic for you, click "Cited by" to see newer papers that referenced them. These newer papers will often be more specific.
Explore! There's rarely a single answer to a research question. Click "Related articles" or "Cited by" to see closely related work, or search for author's name and see what else they have written.