Hi, can someone help me on a simple way of attaching a script to play a particular music in the background while playing a level? Where is a good site to download game music so I can attach to Unity?
Also, how to attach sound effects for when I collide in a particular object. Thanks!
Take a look at this. Audio source
So basically you can download free music (Just google royalty free music) or flashkit. And once you have it just drag it into your project. Say you want a particular gameobject say a box to have a explosion sound. Drag the explosion sound from the project to the gameobject. And it will become a component. Untick play on awake if you want to trigger it manually.
To play your audio just put this in a script
function Start()
{
audio.Play();
}
For background music, i think you could attach the music to the camera as there’s a audio listener on that component and keep looping the music. 
Hope this helps.
just to add, for music when you import make sure it is a 2D sound not a 3D sound