To enter Scotland, you must have a passport that is valid for the whole of your stay (and we recommend extending that so that your passport is valid for at least 90 days beyond your intended departure from the UK). Check your expiration date!
If you have a US passport, you will need an Electronic Travel Authorization (ETA) prior to travel in the UK (that includes Scotland). The application is simple and can be done online here. It costs £16 to apply online or through the UK ETA app. An ETA lasts for 2 years or until your passport expires, whichever is sooner. You can travel to the UK as many times as you want while you have an ETA.
If you are a citizen of another country you can check visa requirements here.