Replying to Film trouble

ya this might be a stupid question and it might be in the wrong place but i made an edit a while ago on my regular like 1 year old mac book. we filmed with an hd camera with imovie hd and it was good quality until i was finished and i condensed it. After i did that the version i could put online was wicked shity like terrible quality and i dont know what i did wrong.
 
it would be much appreciated if i could get some pointers
but no need for hate