Can I put just 10,20,30 seconds of a song in my movie?

It's all or nothing sadly.

What you can do is duplicate your project, then delete everything you don't want music on.

Add the Music and export the video to the camera roll.

Then import that video into your final edit, and it will be the only part with music.

This is how I do it:

In Clip Settings, turn Loop Background Music OFF

Add song to your Project

Trim Song by dragging right handle to the left, and/or vs

If you need to have that portion of song in a different point in Timeline, then

Add another song to start of Project and mute its volume

Trim the first song by dragging right handle towards the right and adjust its length to the point in which your second song suppose to start.