Since you're new to the game, you should start by watching arrayseven's playlist on how to BE the best medic you can, for some visual guides and mentoring. Linked here!
After finishing the playlist, you should be ready (and hopefully excited) enough for some paperwork!

I personally recommend this over arrayseven's videos, but why not see both! (not like you have anything better to do anyways :/).

The Medic's community strategy guide (and for pretty much every class) in the TF2 wiki is the BEST source of knowledge and tips given to you by not only the developers of the game themselves, but the best of the best competitive and casual players in the history of this game!

Now you know why I prefer this over arrayseven's videos ;)

You can enjoy reading the Medic's community strategy guide here!

Gosh I sound like a salesman...

Don't forget to have fun and be prepared for some love letters from your team :D.

Hope that'll be a great help for you even if you don't plan to main medic :).

Much love!

-Moody