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?
I like The Heart of the Buddha's Teaching by Thich Nhat Hanh as a beginning book on the core of the Buddhist teaching. It is well written and modern in style, and covers essentials such as the Four Noble Truths and the Noble Eightfold Path as well as slightly deeper topics. It will give you a grounding and a place from which to appreciate the sutra's. 

There are various web based repositories of sutra's which you can explore, I found http://www.accesstoinsight.org to be pleasant and good. The book includes many references to sutra's which you can look up online. 

One thing to be aware of in Buddhism is that the different schools hold differing opinions on the words of the Buddha. There is a doctrine called the Three Turnings of the Wheel of Dharma, which sets out the Mahayana view on the Buddha's teaching, while the Theravadan view is that the Pali Cannon which constitutes much of the Mahayana tradition's First Turning is authoritative by itself.

The Tibetan tradition has composed a series of texts called lamrims, of which there have been many following on from Atiśa's 11th-century original text A Lamp for the Path to Enlightenment. These aspire to order the sutra's in a logical order of progression for actual practice.