You're correct that "katotohanan" means truth. It's a noun form of the adjective "totoo", or true. The phrase you're looking for can be translated at least two ways: "Katotohanan ang sinasabi ko sa iyo" would be a literal translation. You could also say "Totoo ang sinasabi ko sa iyo", which means "what I'm telling you is true". The difference is subtle enough that both sentences can be used interchangeably. Yes, your last sentence is correct. For me, it seems more correct if you add na (now or already), as in "Alam ko na ang katotohanan", but that could just be a personal preference :/

Nagsasabi ako ng katotohanan. - im telling the truth. Alam ko ang katotohanan is correct. :)