let (someone or oneself) in for
To involve someone or oneself in something, especially that which proves to be more intimidating, demanding, or overwhelming than one was expecting. I hope you know what you're letting yourself in for with this case—it's going to drag your reputation through the mud for a while. Just what have you let me in for, Tom? I said I wanted to earn a little bit more money, not get involved with the Mafia!
let oneself in for something
to make oneself vulnerable to some difficulty. I don't want to let myself in for a lot of extra work. You really let yourself in for some problems!