when your standard operating procedure or hazard assessment calls for an apron, an apron is necessary. You may also deem an apron necessary or desirable if you believe there is a change of spoiling good clothes or soiling your lab coat, or are undertaking work with a volume of liquid which is large enough and hazardous enough to damage clothing.
Again, see your standard operating procedure or hazard assessment for the requirements. An ankle-length apron is good protection from large volumes of extremely dangerous liquids such as human pathogens, hydrofluoric acid, and very hot liquids.
The donning and doffing of an apron is typically intuitive. The photo shows a common style of lab apron and how it is worn. Visually inspect the apron material before putting it on and be certain that there are no tears or holes. Do not use an apron that is not completely intact. If the apron suffers splashes or other chemical/biological contamination during use, contaminants should be cleaned from its surface before or immediately after removing it.
Obviously the apron does not protect what it does not cover. Wear suitable protection on the upper parts of your body. Aprons are made of a variety of different materials, some of which may be less appropriate for than others for particular chemicals. Know (read manufacturer's information) what types of chemicals your apron will and won't protect against. Remove the apron as quickly and carefully as you can in the even of a drenching accident. (In case of drenching it will be necessary to assess, very rapidly, whether you should remove the apron BEFORE or AFTER you get yourself into the emergency shower.
Read the manufacturer's information for best care and maintenance instructions. Aprons should be kept clean and stored in a dry, temperate storage area (hung or folded). No protective equipment that has been contaminated with hazardous materials should be taken home for cleaning. Any item (including an apron or lab coat) which is so grossly contaminated with hazardous material as to qualify as hazardous waste must be disposed of as such. (This is a call that might be difficult for you to make; the criteria for identifying a waste as hazardous waste are extremely bizarre. See "when is a waste a hazardous waste?" Or contact the REM hazardous materials section at 40121.) Aprons (and other items) which are contaminated with biologically hazardous materials, and which you cannot or do not wish to clean, should go into the biowaste.