These are websites and videos with very good information about the science behind global warming.
General Websites:
The Climate Change Resource Center has a large selection of videos on many topics: http://www.fs.fed.us/ccrc/video/
CLEANet has lots of material suitable for high school level: http://cleanet.org/
Focused on the USA, but still a very good source of information: http://www.climate.gov/
Climate Central is also focused on the USA, but is also a very good resource: http://www.climatecentral.org/
Deep Climate is a blog run by climatologists for exposing disinformation and lies spread by global warming denialists: deepclimate.org
Basic Videos:
NASA has a 6 minute video explaining the basics: http://climate.nasa.gov/warmingworld/
Another NASA video with the basics: http://www.nasa.gov/audience/foreducators/topnav/materials/listbytype/The_Influence_of_Global_Warming_on_Earths_Systems.html
A 1 hour seminar: http://www.youtube.com/watch?v=VI9W2xPfFUA
A 12-part lecture series (total time ~2 hours): 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12.
On the debate on global warming completely manufactured by the media and nonexistent in scientific circles: http://www.youtube.com/watch?v=gDM3T0-o3r0
Potholer54 is a Youtuber who debunks global warming denialist arguments: http://www.youtube.com/user/potholer54; example video.
Specific Videos:
The American Museum of Natural History has this video focusing on the effects of global warming on the boreal forests of Alaska, to see the impacts on ecology: http://www.amnh.org/sciencebulletins/?sid=b.f.alaska_greening.20100712
For an example of climate change’s effect on a single geographic area, check out this video on the future of climate in Wisconsin, USA: http://www.wicci.wisc.edu/climate-change.php
A TED talk by photographer James Balog showing the reduction in sea ice over time: http://www.ted.com/talks/james_balog_time_lapse_proof_of_extreme_ice_loss.html
Polar bears and climate change: http://www.youtube.com/watch?v=axRX9_UyUOc
Climate change and waste treatment, a 1 hour seminar: http://www.youtube.com/watch?v=15S4teA2l-M
For insights into methods, this is the video accompanying a scientific paper: http://www.youtube.com/watch?v=5UWCj3JFsZs