When an image is downloaded, besides the corresponding bands (which contain the data), you will notice that there are two txt files that accompany the data. Such files contain the Metadata necessary for the calibration of the image. The procedure you should follow is described in several papers, among which Chander et al. (2009) is highly recommended when using Landsat imagery. Next we describe the procedure we recommend to accomplish this task. The following is an example with a Landsat ETM scene, prepared by the ecologist and agricultural engineer C. Buitrón, under the assessment of the geographic engineer J. Fernández. Both are independent consultants in environmental projects.
Remember that in case you wish to carry this procedure to calibrate TM data, you should follow the procedure suggested in Chander et al. (2009), and generate the equations to convert DN to reflectance (perhaps using ERDAS´ Model Maker).