while some people are born a medium, some can train to become one, and some just don't have the ability. not everyone can hear spirits, just as not everyone can sense, smell, or see spirits. also, because you can see spirits, doesn't mean you can see demons or astral beings and vice versa [you can see demons, but you might not be able to see spirits] i'm just saying these things so you don't get too disappointed if all you can do is sense spirits or something.
you should balance your chakra's to have an unobstructed flow of energy in your body. start from the root and work your way up to the crown. different crystals, herbs and items can help you open the easier. try and study each one before balancing them, then balance one a day for 7 days so you can feel the difference each one will make in your life. the day before you start, go about your day and write down everything you feel, if you feel nervous, if you talked a lot, got a sudden headache and so on.
mediumship comes from your sixth senses [psychic abilities] so you should look into difference exercises to try and strengthen your premonitions and psychic side. some teas can also help with this, but be sure the herbs aren't poisonous, you aren't allergic, and the tea is suppose to be ingested. some potions [teas] are meant to be smelled, not drunk. while you work on strengthening your abilities, try to also study protections and cleansing to keep safe.
when you wish to communicate, you can try meditating, once in a meditative state, ask questions and listen for an answer, it could be verbal [though it won't sound like your voice] could be a feeling, might also be a sound, like a tapping, or something knocked over. pendulums, skrying mirrors/balls/water, or automatic writing can also be used to speak with them. for each of these methods though, be sure you're in a meditative state.
keep a clear head when dealing with spirits, it's true spirits can and do lie, but you shouldn't instantly conclude it was a spirit that made a sound, or whatever. look for logical answers before spiritual. i know it's hard when you want something to happen, but strengthening your clairvoyant side takes time, and communicating with spirits is a long process, so take your time.