To become a psychic medium it makes sense to familiarize yourself with the advantages and disadvantages first.
The advantage is you might get information you wouldn't be able to get otherwise, and that you might develop a relationship with a non-physical entity. The disadvantage is you are playing with fire - some part of your energetic circuit will be 'burnt out' to allow the higher flow of psychic energies to circumvent your natural psychic defenses.
How to become a psychic medium? The first requirement is to desire (or need) that information so much that this risk becomes acceptable to you. The second requirement to become a psychic medium is to have the willpower to actually go through with it.
Where to start? The best way is to find a capable mentor whom you trust and are willing to follow. If that is not an option, practicing automatic writing might be a decent, less risky way to start. You can become aware of the interaction of your conscious and unconscious processes, and later on you can feel and decide on the 'texture' of psychic energies you wish to let through yourself.
Becoming a psychic medium can jumpstart your psychic or spiritual progress, but it is not really a shortcut - even with help, you will have to travel the path yourself.