Hi, how do I preview transitions between images and videos?

[Lava9]

I am also hoping to have music play in the background. If I include the music composition as my first element in the timeline panel, does that mean the song will play while the images and video clips play? Thanks.

[Rick Gerard]

After Effects is a "visual effects" and motion graphics app designed specifically for and suited only to creating shots and short sequences that require processing that cannot be done in a nonlinear editor like Premiere Pro. It is about the worst video editing app out there, but it is a very capable tool for creating visual magic you cannot create or composite in your NLE. I've been using AE for a little more than 25 years and my average comp is one shot under seven seconds with somewhere between 10 and 50 total layers that are all needed to make something look like it was there when it really wasn't.

If you want to play music in the background while you edit video and still images to the music AE is NOT the tool of choice. You should be using Premiere Pro.

If you insist on trying to use AE for that kind of work then spend a couple of hours in the Beginner section of the User Guide so you will have some idea how the UI works and what a standard workflow is. You will need one layer for every different shot in your movie.