The lights and appliances in my house go dim and weak when I turn on the window air conditioner.?

I've been told I need to add more electricity to my house by having something stabbed into the main electrical source. I don't know what this is or how to explain it to an electrician. Is this an expensive job? Sorry to sound so dumb, but I'm a widow and know nothing about such things and I don't want to be taken to the cleaners by an electrician telling me to do things that I really don't need. Can anyone help?