How to use KissFFT with audio?

Question

I have an array of 2048 samples of an audio file at 44.1 khz and want to transform it into a spectrum for an LED effect. I don't know too much about the inner workings of fft but I tryed it using kiss fft:

kiss_fft_cpx *cpx_in = malloc(FRAMES * sizeof(kiss_fft_cpx));
kiss_fft_cpx *cpx_out = malloc(FRAMES * sizeof(kiss_fft_cpx));
kiss_fft_cfg cfg = kiss_fft_alloc( FRAMES , 0 ,0,0 );

for(int j = 0;j<FRAMES;j++) {
    float x = (alsa_buffer[(fft_last_index+j+BUFFER_OVERSIZE*FRAMES)%(BUFFER_OVERSIZE*FRAMES)] - offset);
    cpx_in[j] = (kiss_fft_cpx){.r = x, .i = x};
}

kiss_fft(cfg, cpx_in, cpx_out);

My output seems really off. When I play a simple sine, there multiple outputs with values way above zero. Also it generally seems like the first entries are way higher. Do I have to weigh the outputs?

I also don't understand how I have to treat the complex numbers, I'm currently using my input values on the real and imaginary part and for the output I use the abs, is that right?

Also usually spectrum analyzers for audio have logarithmic scaling, so I tried that but the problem is that the fft output as far as I know isn't logarithmic, so the first band for example is say 0-100hz but optimally my first LED on the effect should be only up to like 60hz (so a fraction of the first outputs band), while the last LED would be say 8khz to 10khz which would in that case be 20 fft outputs.

Is there any way to make the output logarithmic? How do I limit the spectrum to 20khz (or know what the bands of the output are in general) and is there any other thing to look out for when working with audio signals?