The BMJ has made a classic series of papers by Prof. Trisha Greenhalgh on the subject of how to read and criticise scientific publicatuions available for free online. There is a strong emphasis on medical research but many of the principles are the same for any scientific discipline. The collection can be found here: please note that for some reason they're in reverse chronological order so I recommend that you begin with "The medline database" (at the bottom of the list) and work upwards.
