iMovie is ideally suitable for this. Don't worry about converting to an mp4, there is no need.
Open up iMovie and create a New Project. Into that project import a video clip or a still photo/picture. To remove/turn down the volume of your video clip, move the mouse over the clip and a little gear wheel symbol appears at the beginning of the clip. Click on that, and select Audio Adjustment, then drag the volume slider to zero.
If you use a still photo/picture you will have to make it show for as long as your music track. Again, move the mouse over the picture and click on the gear wheel. Select Clip Adjustments. Click in the Duration window and enter the time of your music track. Note: 3 minutes 20 seconds needs to be entered like this: 3:20:00
Now to add the music. You can either drag it direct into the Project from a Finder window or from iTunes using the Media browser. Click on the music note symbol on the right hand side of the iMovie window to open that. Either way, if you drop the track on top of your clip/picture it will become a separate track in it's own right, showing as a green bar under the clip. If you drop it in the general area, the whole background turns green, and it becomes a background track, where you add further sound/music on top if you wish.
If you've finished, you can load direct to you YouTube account using the Share menu. Some people have some problems with this, so you can also export to Quicktime, again using the Share menu and upload via the YoutUbe website.