How to get my movie/video clip onto my iPhone through iTunes?

So basically I want to get this file onto my iTunes to get onto my iPhone. The file was originally an.avi file then I renamed to.mp4 but it still isn't letting me add to library. Help As soon as possible? (:

Thanks!
I have a 4S by the way and I have 3 short videos and 1 movie on there already if that effects it in anyway!

It depends what was in the AVI container to start with. If it was not a variety of MP4 you will need to convert it. Try the free MPEGStreamclip…

http://www.squared5.com

and convert to an.m4p file.

You can't just rename the file extension from "avi" to "mp4". You have to convert it. Use VLC Player and get it here:
http://www.videolan.org/vlc/
Yes it is free and it will play just about any file format and can stream and convert most formats too.

Once it is converted you need to import the file into iTunes and then click and drag it to your iPhone into the video folder (it doesn't isn't really a folder but to the word " Videos") and do this while your phone is connected to your computer.

Do a Youtube search for how to convert with VLC and how to copy to your iPhone. There should be plenty of videos that will help you to understand and figure it out.