Cant update to imovie!

I got a MacBook air for Christmas and ever sense i've gotten it i wanted to download IMovie so i can edit my youtube videos, but it says "you need to download the new update", and so thats what i did, it restarted but when i came back it say "some updates couldn't be installed" so i tried it again, again, and again. to this point its December 31 2018. and it still said some updates couldn't be installed. Please help.