How do I create an alert on SharePoint 2013 for when the contracts are about to expire?
You can create a SharePoint Designer workflow.
It's an ok solution if you don't have many items or if you need to pause for a short time. Otherwise keep in mind that every workflow is running until the email is sent.
Another solution you could check out is Information Management Policy (library>settings). It allows you to e.g. delete items after a month or two, or e.g. start a workflow after given time. Depending on the size of your library/list, it may be more optimal than pausing an SPD workflow.