I don't know much but here's my answer anyway. A solid's molecules are tightly packed together, making it hard form them to move. Therefore, their shape and volume always stay the same. Hope that helped! :)
Every atom of solid or any other substance is constantly in vibration. This is due to the thermal energy in solids. This vibrational energy is also very low in the atoms or molecules of solids. All these factors give a definite shape to a solid.