I've watched basically every environmental documentary out there, and I recommend educating yourself as much as possible, but this shortlist is all of the documentaries that changed my mindset massively and helped to inform my opinion on many things.
Gasland directed by Josh Fox was the documentary that got me to start using my voice as a tool to inspire environmental change. After seeing it I became very involved in anti-fracking activism which helped me to become trash free! A must see.
King Corn follows Ian and Curt as they try to learn where their food comes from. To do this they end up planting a hectare of corn. The process of doing this and what they end up with was one of my main motivators for becoming a vegetarian.
Food Inc. - must see.
No Impact Man - This documentary follows Colin as he lives without making an impact (this isn't totally true as he still uses fossil fuels) but it's a pretty cool doc.
An Inconvenient Truth - The climate is changing.