Some journals, such as Evolution and Journal of Evolutionary Biology, are primarily devoted to evolution, whereas others, such as Nature, Science, Proceedings of the National Academy of Sciences, and PLoS Biology, cover biology in general. Many journals publish on evolution as it relates to their primary focus (e.g., Ecology Letters or American Journal of Human Genetics). A substantial literature on evolution can be found in surprising places (e.g., Institute of Electrical and Electronics Engineers Transactions or Physical Review Letters).
Each journal has a characteristic niche, reflected in the kinds of papers that it tends to publish and in their style. Sometimes, the subject area is obvious from the title, but often journals have particular biases (e.g., Proceedings of the Royal Society (London) B tends to publish on sexual selection and ecological genetics). Articles vary greatly in accessibility. Those in high-profile journals such as Science or Nature may be highly condensed, with crucial methodology tucked away in Supplementary Information. However, there may be helpful commentaries associated with such papers. Papers in journals such as Evolution and American Naturalist may be longer, with a better overview of the existing work and context.
The best way into a subject is to read a good review. Many more reviews have been published in the past few years, but they vary greatly in quality. Annual Review of Ecology, Evolution and Systematics is perhaps the leading review journal; Annual Review of Genetics, and Annual Review of Genomics and Human Genetics also have relevant articles. Trends in Ecology and Evolution has more frequent and shorter reviews, which may be more accessible, but are generally less comprehensive. A good paper will review the relevant literature in its introduction.
Papers in the journals listed here are peer-reviewed. For information on peer review, see Sense about Science and Wikipedia. Usually, two researchers in the same field will read the manuscript and send anonymous comments. They may recommend outright rejection or extensive revisions. This process helps to keep papers accurate, but it is not foolproof. Bear in mind that no one source of information can be entirely reliable, and so where possible, take the time to critically compare multiple sources.
We give a rough classification of journals in the field here.