This means to enter into something or make a beginning.
as we know in place of one's we use `my, his, her, their, our` etc.
In place of `to` we use noun or pronoun as a subject of sentence.
I had a very good interview in the company yesterday and I am sure i have got my foot in the door now.
Any interview is a good opportunity for us to get our foot in the door.
By improving our English we can get ready to get our foot in the door.