Skeptical Science. This is the go-to site for answers to questions about what is true and false in the confusing and conflicting welter of information & misinformation about the climate problem.
Realclimate. This site is run by working scientists both for themselves and the interested public. It reports on recent discoveries and controversies, as well as offering refutations of false claims.
Yale Environment 360. This is a publication of the Yale School of Forestry and Environmental Studies, with excellent articles on many subjects related to this problem
The Discovery of Global Warming. This superb site is a web version of Dr Spencer Weart's book on this subject. In fact, it has much more than the book, is very well organised and a mine of information.
NASA Goddard Institute. One of the world's leading research institutes. There is a link here to the page of Science Briefs, short articles by GISS scientists on their work; and one to the GISS temperature record data.
James Hansen's site. Hansen is thought by many to be the finest scientist working on this problem. He retired as GISS director in 2013 to concentrate more on advocacy. All his publications are freely available here.