You should ask your librarian - book an appointment with your subject's subject librarian and he or she can take you through the process.
Maybe there are other ways but basically what you are looking for is the 'h' index. Try (if your uni has it, and it should) Web of Knowledge, found under your library's databases. I think it is now known as Thomson Reuters but either should get you there.
If you are just at the beginning, believe that you will over time get to know the key journals in your field. Also don't forget your supervisor, that's part of their job, to advise you, not just provide critique ;-)