You need to provide more details! There are many "satellite's". If the data is 0-255 it is 8-bit and represents DN. In a processing workflow some analyst prefer to scale floating-point to 16-bit so, bit depth does not always reflect correction level. Since you downloaded the data from Earth Explorer and it is 8-bit it is certainly DN.
I do not know what you mean by "radiation". The common correction is top-of-atmosphere-radiance and then at-sensor-reflectance. The necessity of this correction is really dependent on what you are planning on doing and how clean the image is. Any time-series comparison requires correction to at sensor reflectance, preferably with an atmospheric correction. There are other caveats as well. For example if you want to calculate a tassled cap transformation on ETM+7 the coefficients are specific to reflectance values but TM5 is calculated on DN's. If you are performing a classification or calculating indices on a single date image then it may not be necessary to perform this correction.
As far as the "expression" to apply to the data. It is dependent on the sensor. In ERDAS you need to write your own model for the correction. In ENVI there are many sensor models where you can read the header directly and apply a correction on the fly. You will need to read the help documents which, provide detailed instructions. If I am preforming time-series analysis with a mix of Landsat 5 and ETM+7 I will calibrate the dynamic range of TM5 to ETM+7 during the reflectance correction (which is not available in the ENVI TM5 sensor model).
The equations for Landsat at sensor reflectance corrections along with required coefficients are available in the USGS Landsat Science Data Users Handbooks and various other USGS websites.
Landsat ETM+7 Science Data Users Handbook
Revised Landsat-5 TM Radiometric Calibration Procedures and Postcalibration Dynamic Ranges
Landsat 8 at sensor reflectance corrections
You can easily research this on your own with a simple Google search. If you are using ASTER it is a bit more complicated and you will need to dig into the literature for the equation and associated coefficients. If you get stuck, contact me and I can provide ERDAS graphic models for ETM+7, TM5 calibration and ASTER. Although, it would be nice to see a good faith effort on your end. This site is intended to help you through problems and not just do your work for you. Based on the scope of your question it does not look like you have put much effort into researching basics.