Latvian Ancestry - need help

Hi all

I am from South Africa, My Great Grandfather was born in Latvia in 1881, and I want to find out if I qualify to get citizenship, as I would like to make the move over. According to the research I have done, I qualify?

Documents that confirm the eligibility of acquiring citizenship based on ethnic origin:
1.   Document that confirms ancestor’s place of residence in territory of Latvia between 1881 and June 17, 1940;   
2.   Documents proving relationship to the ancestor;
3.   Documents confirming ethnic background (passports, birth/marriage certificates etc.)
4.   Confirmation of Latvian language exam being passed.

And if so, where do I start?