i'm not too sure about becoming a medium, but i can suggest a few ways to strengthen your psychic abilities.
while meditating, open your third eye, visualize purple light/orb/disk/flower [people picture different things, i use a violet gem] focus on the gem, it may appear dirty, or you may not be able to see the colour very well, focus on it, visualize it getting clean.
make 3 predictions about your day, don't go for normal things, something like 'mom will make lamb stew' or 'my best friend will be wearing a new yellow shirt' write them down and check them off if they happen. don't just guess, normally while meditating they should come to you.
before answer the phone, try and predict who's calling. this doesn't work with me anymore, since only the same 3 people call me lol.
take a deck of cards [normally they're special cards, but you could use tarot or playing cards] draw a card and try to visualize what card it is.
once you think you've gotten the hang of your abilites, try meditating in a cemetary, see if you can sense/hear anything. [i choose a cemetary cuz it's the most likely place to find a spirit, but if you know of a place where there are spirits, go there] psychic ablilites mostly take focus, it get easier as time goes on, but focus and practice is key.