If you are new to AE you need to get familiar with the basics first by going through a few of the beginner tutorials in the User Guide.
The blue animated background can be created with Fractal Noise + color effects + Glow. The layer is then masked and put behind the front layer with the animated character.
The next step is to add the audio track, convert audio to keyframes. check out the "Both Channels" property in the newly created Audio Amplitude layer using the Graph Editor and looking at the average values for the peaks and valleys of the value graph. You make a note of those values then apply it to Scale for both the blue fractal noise layer and the character using a linear method expression that looks something like this:
t = thisComp.layer("Audio Amplitude").effect("Both Channels")("Slider");
tMin = .5; // minumum value of Both Channels
tMax = 19; // maximum value of Both Channels
value1 = 80; // minimum scale value
value2 = 130; // maximum scale value
s = linear(t, tMin, tMax, value1, value2); // linear interpolation from expression language menu
[s, s] // array assigning the value of 's' to both X and Y
I already showed you how to use time remapping to drive the particles on the bottom layer.