I can't find any white tshirts who are thick enough to hide underwear and skin

Welcome to the wonderful world of camisoles. Amazon sells 'em. Just buy white cotton ones, as plain as you can find. On hot days, even though it's an added layer, it should still help re sweat.

If your nips are showing, that's not a t-shirt problem, that's a bra issue. A more padded bra in a nude/neutral tan color will blend in with flesh tones more than a white bra will.

Also, purchase a better quality of t-shirt from Haines, Fruit-Of-The Loom or Under Armour. White isn't always thin but cheap brands are.

Hm.... I have some tshirts from Fruit of the Loom, they are advertised as Heavy Cotton but they feel very thin, luckily they aren't white.

My problem is the white tshirts are so thin that they show my black bra.
They need to be thicker. I used two white tshirts and got a 100% better look but they are still two and not one. Where can I find tshirts who are really thick like two white Tshirts or even 3 white ones?

Why would you wear a black Bra under a T-shirt? Invest in a neutral color or paint your skin a dark color to match your black bra. Sheesh!!!!

Search for "Beefy Tee". Those are usually pretty thick.