In your sentence "which/that I want to talk about" is essential information. In other words, it is necessary to include this clause because without this clause the sentence is meaningless. "Friendship is the topic." is a meaningless sentence - it doesn't tell us that it's the topic you want to talk about. Clauses that are essential/necessary are called "defining" “identifying” or "restrictive" clauses. Thes clause can take either "that" or "which." For example: "There aren't a lot of companies THAT/WHICH are hiring now." This is a defining relative clause because the information is essential -- if you take away the clause, the sentence says "There aren't a lot of companies" and that isn't right. So, there's your answer... "Friendship is the topic that/which I want to talk about."