These are real AE basics you're asking here. Composition Settings dictate the length of the timeline. If you create a new composition from your video the comp will exactly match your video in every way. Frame rate, frame size, and duration.
Your RAM preview will be limited to the amount available RAM you have. Please see this from the FAQ.
It is next to impossible to learn how to use After Effects without some training. Please start here:How to add effects, motion graphics in After Effects | Adobe After Effects CC tutorials
thanks for the quick reply! i figured they it was a pretty nooby question but hey i can only go up from here right?