The selection of activating agents for the thermochemical creation of high-grade activated carbon (AC) from agricultural residues and wastes, like feedstock, requires innovative techniques. Overcoming energy losses, and using the best methods to minimize secondary contamination and increase absorptivity, are critical. The present review focuses on the study of the synthesis and characteristics of low cost activated carbon. Various investigators focus on physico-chemical characterization of AC prepared from waste and low-cost materials using TGA, FTIR, XRD, and SEM techniques. Physical properties like bulk volatile matter content, moisture content, density, surface area, ash content, porosity, apparent density, acid porosity, iodine number, fixed carbon, soluble matter, and pH were investigated and analysed in detail by various researchers. Here, we also review the importance and influence of activating agents like H3PO4, KOH, CaCl2, ZnCl2, and H2SO4 on agricultural waste and discuss some of the methods for the preparation of AC. These activating agents are used by various researchers for activation of carbon prepared from waste and performed at various conditions. The review also covers the adsorption isotherms that were developed by various researchers in their work.