i did this in history as part of an annotated bibliography... if you're unsure about how to go about it, cover a few of the following points:
what did you use the source for [simple background research, or comprehensive information on someting etc]? was it useful/effective as a source? was it easily accessible or difficult to navigate? how does it compare to other sources you have used? how good (accurate/reliable) was the information provided i.e.
wikipedia - this source has a tendency to be unreliable hence it was used only for the purpose of gaining background knowledge on the topic. However it was effective in this role.
really good site - this site was effective as a source as the information it provides on [insert point here] was accurate and comprehensive. however it does not include/there is less information on [insert point here]
so on and so forth. good luck