Dr Martens Stretch and Soften

If you can't get them on or they are very tight
Take them back and get a size up or a different model.
Base of tongue
Glue the loose bit of lining down and stretch the boot up at the base of the tongue.
General
Please note, this is one of those jobs that doesn't help as much as you would like!
If you can return them for a bigger size. Do it.
Give an overall stretch. Spray with leather softener. Coat with leather lotion.
Don't let your new Dr Martens rub you raw. We can stretch and soften them to help you get wearing them in style and comfort.
Once you get them back. Wear them for a few hours at a time to avoid hurting your feet.
If still a bit tight consider wearing with out the insoles for a few days till you get them worn in.