How do I project a map onto a TV that I can reveal as the players go along?

I have D&D Beyond, but can't figure out how to project the map onto a TV that I have laying on a table that doesn't show the whole map. I see the fog of war feature but I've played around and don't see how it works.

I'm being told that Roll20 is a good alternative, and that Foundry may be worth the purchase since I'll be doing this long term. But, I have all of the CoS maps on D&D Beyond. Would I be able to get all those maps on one of those two alternatives?

Any help is appreciated. I'm in the dark here. Please explain it to me as you would a young child, or a Goldne Retriever.