We actually had a very extensive thread about Rwanda just two weeks ago: right here. You should especially check out EsotericR's answer, which answers your first question in great detail.

To give a small summary: Hutu and Tutsi already existed before the Belgians came to Rwanda, but they were fluid socio-economic groups rather than ethnic groups. A Hutu could become a Tutsi and vice versa. The Belgians transformed the concepts of Hutu and Tutsi to two ethnic groups and introduced the narrative of one group as invaders - which all led to the genocide.

As to responsibility, that's a difficult question. Many historians don't like to talk about responsibility - they just leave the implications unspoken. Many historians don't like to make judgement calls about history. I think it's fair to say, however, that Belgium is at the very least indirectly responsible for what happened. The events after the colonial era played their part too and the people who committed the genocide are ultimately responsible, but at the same time, so is Belgium.