Unfortunately, controlling a straight set of Christmas lights isn't an entry-level project.
Entry level projects are built around the idea that mistakes are an inevitable -- even desirable -- part of the learning process, so they're designed to keep the mistakes harmless. Christmas lights use wall power (120v AC in the US), and that has a challenge rating of: "can kill you".
It takes time and experience to get used to working with wall power.. what kinds of conductors, connectors, and safety practices to use.. and then more time and experience to learn how to build circuits that do more than just "turn the light on" and "turn the light off". At best, the mistakes will blow a hole through your favorite set of wire cutters, and at worst will burn your house down around your family while you're sleeping.
Beyond that, the Arduino is a low-power device. Using it to control wall power is a bit like using a mechanical egg timer to steer a pickup truck.. yes, it's possible; no, it ain't simple.
That doesn't mean you're barred from enjoying some blinkenlights though. It just means you need to adapt the circuits to work in the 'safe' zone. In general, that means chopping the string of Christmas lights up into pieces that will work for a supply voltage of 5-12v DC, and controlling each segment separately.
When you void a product warrany, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.