Hello Shelley,
Venus's hot surface temperature of 750 Kelvin is a result of the greenhouse effect.
Have you ever gone out with your parents on a hot summer day and left the car in a parking lot, only to come back a few hours later and discover the inside of your car is extremely hot? Do you know what happened to make your car so warm? Well first sunlight entered through your car windows. The radiation was absorbed by the dashboard and the upholstery which raises the temperatures. Every object emits blackbody radiation appropriate to its temperature. Wien's law relates the Kelvin scale temperature to the dominant wavelength. Thus sunlight has its greatest intensity at wavelength of 500 nm, in the middle of the visible spectrum. This sunlight typically raises the temperature of the car's upholstery to around 330 Kelvin. At that temperature the seats in your car will reradiate the energy at a wavelength of 8800 nm. This radiation is in the infrared portion of the electromagnetic spectrum, to which your car windows are opaque. This energy is therefore trapped in your car and absorbed by the air and the interior surfaces. As more sunlights comes through the windows and is trapped, the temperature continues to rise. This is known as the greenhouse effect.
Carbon dioxide is responsible for the similar warming beneath the Venus's atmosphere and to a lesser extent the Earth's. Like your car windows, carbon dioxide is transparent to visible light, but opaque to infrared radiation. 96% of Venus's thick atmosphere is carbon dioxide. The warmed surface emits infrared radiation which cannot penetrate the C02 rich atmosphere. All of this trapped radiation produces the high temperatures found on venus.