How to understand Buddhism original dogma?

Buddhism has been evolving dramatically since Buddha created it. Somehow it spawned into many different 'sects' across the world. There are numerous sutra and books on Buddhism. As a beginner, it's really hard to know what to start, but I had a feeling that I need to understand the original dogma/meaning of Buddhism when it was invented. However I heard that most sutra were written by Buddha's disciples, or disciples' disciples, or even laymen; Some of the recordings are contradict each other when interpreting Buddha's true meaning; Some original meaning was lost or probably altered since then. Anyone has suggestion how I should start and what to read?