If your apartment is cold during the winter and you want to warm it up without having a high energy bill then you can do a few things to help keep the apartment warm and lower your energy bills.
Make sure the blinds are closed if you have window blinds. Try putting up some thick curtains to help hold the heat in your apartment.
On sunny days though when you can get some sunlight in through the windows, open your curtains and blinds and let the sunlight warm your apartment.
Get an electric space heater and use it in the room of the apartment you're in instead of trying to warm the whole apartment.