How can you get inside a door that has a keyhole door knob?

The credit card-thing would work if the you are dealing with an older -- not a deadbolt-type -- lock.

My best success with a deadbolt-type lock -- other than calling my son-in-law who is a locksmith! -- is to get two small (flat blade) screwdrivers - one in each hand - and try to work that "thingie" back toward the doorknob. Sometimes this works and sometimes it does not. It depends on the innards of the knob lock.

Drill it out

bump it (google key bumping)

kick in the door

call a locksmith

climb in a window

Sometimes you can use a credit card and slip the plunger out of the striker plate.

Call a locksmith.

I could not. Despite that I am not huge, I would not fit.

Put the key in, turn it and unlock it...and turn the handle...

unlock it

Use the key...