If you are doing community service through an organization, they often have a volunteer coordinator who keeps track of your hours. Don't relay on that though, as computer systems are unreliable. And coordinators change jobs. Keep your own record, check it against the record of the person or office who you will use as the required Contact to attest to your involvement. Ideally, they will match your records.
The AMCAS application asks for start month and end month and total hours. If the involvement is intermittent, like only during the school year, you can include up to four timeframes per activity. For the contact you need the name, title, and either a phone # or email address. You have 700 characters to describe the activity. You get 1325 extra characters for the three activities you decide are Most Meaningful to you.