Use the Journal Citation Reports (JCR) available through the Web of Knowledge service. To check the impact factor of a specific journal you can search by title. Otherwise, you can browse by subject (subject categories are brought together in the Science and Social Science strands).
You can sort the information in many different ways - you will most likely wish to rank journals according to their impact factor. In addition to the impact factor you can check other aspects of a journal, such as immediacy index (a measure of how soon after publication the "average" article is cited - useful in comparing how quickly different journals are cited).