There are several ways to evaluate journals' metrics for ranking purposes. One common method is by valuing a journal's impact factor (JIF) to determine its relative strength in comparison to other similar journals in a given discipline. Journals can also be valued by citation impact. Article-level citations are averaged for specific journals to create the citation impact of those journals. Article-level citations are also taken as aggregates to evaluate the impact of specific authors and/or their articles.
To track citation counts by author or article, consult any or all of the following resources:
Google Scholar Citations (use to claim ownership of your published research indexed in Google Scholar, track citation counts)
A very detailed and useful guide for citation counts, journal rankings, and more.