The Alert Me feature found in SharePoint is a great way to receive automated alerts regarding documents, lists, etc. 
I will you walk you through setting up an alert in a document library. Go into a current Document Library you have.  Choose a document that you want to be alerted on and click the dropdown on the right side of it.  Select Alert Me.
 
You are now at the Alert Me criteria screen.  The first action item is creating an Alert Title.  This is what will show in the subject of the email notification.  
 
Secondly, you need to decide who you want to send the alert to.  You can choose yourself and others as well.  If you are unsure of their Active Directory accounts, you can use the  <- Check Names or Look up Options
 Next, you need to tell the Alert Me what criteria changes you want to be notified on.  There are 4 options to select from so you immediately have different options that will vary depending on the situation.
 
Finally, you get to choose when you are alerted.  I find this to be the most important aspect of the Alert Me function.  This helps all of us with keeping our Outlook Inbox under control.  You can be alerted immediately, daily, or weekly.  If you choose weekly, you actually get to choose the day and time of when to be alerted!
 
Click OK and you are now on your way of keeping tabs on documents within SharePoint.