first you might want to read this forum Documentation , and learn How to post a question
read the ffmpeg documentation
it is a VERY VERY HARD program to build
-- very hard
on a scale from 1 to 10 -- it is a 9.5 in difficulty
Also what operating system are you using ?
what version of GCC ?
have you built or installed ALL
of the prerequisites that ffmpeg NEEDS
what have you done so far ?
where are you stuck ?
and the rest of the 20 questions game ....