Educate yourself on the science behind studies of global warming. This is not an issue based on religious belief or political affiliation, so you cannot base your conclusions on these latter criteria.
Consult authoritative scientific resources to get the facts on global warming. These include respected popular magazines that specialize in science, academic science departments and internationally recognized scientific research institutions. Ultimately, peer-reviewed science is the best source for the most up-to-date and scientifically valid conclusions on global warming.
Learn what agendas people may have on either side of the debate. Are people having an honest conversation on the topic, or do they have a lot to gain or lose if certain policy decisions are taken based on considerations of global warming? While people may be understandably concerned about policy implications, it is one thing to have an honest debate on policy options, and another thing altogether to misrepresent science to fit an agenda.
Do not be swayed by public opinion on the issue. Public opinion has no bearing on whether a theory is scientifically correct. Centuries ago, most people believed the earth was flat and at the center of the universe. That did not make their beliefs scientifically valid.