I have seen many WW2 documentaries but would like to learn a bit more about WW1.
EDIT:Thanks everyone!
The Great War from 1963 was an excellent 26 episode BBC production with help from The Imperial War Museum. It is relatively old but that meant that they could talk to veterans who were still alive at the time. It also means that the series is relatively cheap and may be up on YouTube.
Watch "The First World War," which was made in 2003, and all ten parts (fifty minutes each) can be found on youtube. It's a fantastic documentary, and shatters numerous myths around the war's origins.
FYI, you will find a few more recommendations for WWI-related docos in this section of the FAQ*. Most the posts contain a mish-mash of topics, so give them a skim for WWI
*see the link on the sidebar or the wiki tab
Gallipoli 2005 is an excellent documentary if you want one that is more specific as opposed to the grand big documentaries that others have mentioned.