I am a newbie, let's get that out first. I set up a comp that I need to place in a wide screen power point slide. The power point slide is set up for a 3:1 aspect ratio. I'd like to fill as much as I can of the slide with the video I am composing. on the comp properties I setup a 3:1 resolution. When I render, how can I tell it to use the comps settings?
When I render, how can I tell it to use the comps settings?
it does this by default and you don't need to change anything.
select the composition you want to render and choose Composition->add to render queue. in the render queue choose "Queue in AME " (as of CC2015.3 this feature is available) this will launch Adobe Media encoder. use the default match source - high bit-rate preset and encode your video to an mp4 file.
more about exporting here: Basics of rendering and exporting in After Effects CC