If I just wear a long-sleeve White T-shirt, is that enough to block the Sun's harmful Rays?

Of course, Anything that covers your skin can protect you from sunburn, but it can still get on your face, legs. I don't think being in the sun covered up like that would be too comfortable. If it's really warm out, the clothes can stick to you because of sweat or heat. Just sit in the shade, wear a sun hat.