The Force is a mystical energy field that all living things have, and can tap into. However, the Force can choose a particular person or family, and those individuals are destined to change the galaxy.

I believe that anyone could be trained to use the Force, but the Force obeys its own rules, so if that person was not chosen by the Force, progress would be more difficult.

Midichlorians are believed by the Jedi to be a good indication of whether a person is chosen by the Force, but even they are unsure of what a high midichlorian count actually means, and if a teacher of the Force is in tune with a person, they can better determine the person's potential than any blood reading.