Don't Put the Cart before the Horse
(don't do things in the wrong order)
Come on, Stanley. Let's go over to the stereo shop. I'd like to buy a big screen TV set and a pair of those tower speakers while they're on sale.
Whoa, there. Don't put the cart before the horse! Didn't you tell me that you were going to be cramped for space in your new apartment?
Yes, I did.
Then don't go out and buy something that may not fit before you have all your other furniture in place. What's more important - buying a new TV and speakers or moving in all the belongings you have already?
I guess you're right. I'd better wait until I see how much space I have.
be cramped = 窮屈な思いををする