Looking for an algorithm that computes "Similarity" between two signals
Coherence? As in: http://en.wikipedia.org/wiki/Coherence_(signal_processing). By comparing the signal spectrums, you get real number numbers between 0 and 1, which each number is a measure of the similarity of the two signals at each frequency.  But it might be difficult for a beginner to understand and program.  You might look for simpler explanations and/or examples by doing a Google search for 'signal coherence', which results in about 17 million hits.