Hello! I am a sophomore at Ohio State University. I am pursuing a major in Psychology and a minor in Studio Art. I have recently shifted my aspirations to attending Georgetown University for a dual advanced degree in Human Development and Public Policy. I am currently involved in one club and have a 3.6 GPA. I will be working in a research lab next year. Any advice on what I can start doing now to better my chances of getting into this PhD program? Thank you so much!! 
Start studying for whatever admissions tests they have you take, and foster a relationship with at least one faculty member who is well known in the field and who can put in a good word for you - particularly for the PhD program. As you are applying for dual degree, you will probably need to apply to each separately and pursue the dual option upon admission to both. It might be easier to start with one first and then work on the other once matriculated, if that is allowed.
Also, for your senior thesis paper try to work both disciplines into your research.
Good luck to you!