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 someone in for something
to cause someone to be involved in something, usually something unpleasant. Fred had no idea what his brother had let him in for when he agreed to take his place in the race. Jack didn't know what he was letting himself in for when he married that dreadful woman.
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!