Brickfilms.com doesn’t directly host the movies; it is more just a place where films can be shared and discussed.
Firstly, you will need a place where you can upload your movie. If you have some webspace somewhere that you can upload your movie to then you are pretty much set. If you do not have webspace then there are some places that provide it for free. There are a couple of options listed on the Resources page of this Encyclopedia.
You might also wish to check with the company from which you get your Internet connection; sometimes they will provide a little bit of space for you (although this is pretty much dependent on what part of the world in which you live.
Once you have your movie hosted on a website, you can start a new thread in the Post & Review Forums , give a description of it and provide the link to where it is located. It is generally considered a courtesy to post the filesize of your film and as well as its duration.