mask in photo shop but i can't find any guide masking in after effect
so what the heck i'm writing the guide myself

first load your source video and put it in the composition

then choose auto trace in the layer menu

in the option choose 'luminance' then change threshold to 88%

this is what comes out when you pressed enter

remove the part that you don't want to use in your amv using
selection tool

after you remove the unnecessary part open up
masks in timeline and remove the unwanted part again
(this time select the mask and delete it)

after removing the unnecessary masks
choose add or subtract the part you want to use
oh and if your mask look dirty change mask expansion by +1 or -1
using this way you can mask a lot's of amount in relatively short time