Short answer: very difficult. The best thing to do, is to remove it directly from the device. How? Well Apple doesn't make it easy for you, because you would basically break their security, however, one program that REALLY WORKS is Tenorshare 4MeKey, it does it in a matter of seconds and is super easy to use, you just have to have the device, a computer and a USB cable that can transfer data (almost any one these days).

If it's yours you must remember a phone number you entered, well, hit forgot my password and try typing in that number, they'll send you information on how to unlock it.

If you don't have any information then that leaves a lot of things in question. They can't be unlocked by anything, not by taking it to Apple or anywhere else (assuming it's locked with iCloud).