"I don't know what to do with myself" is a common English idiom which just means that you don't know what you should do. Use this phrase when you have free time and you're not sure how to spend it:
Now that I'm retired, to be honest, I really don't know what to do with myself.
You can also ask someone a question like this:
So, what are you going to do with yourself over summer break?
This phrase sounds colorful and friendly.