First of the fact you made this post is evidence enough you are motivated to become better, so that's already a really good start. There's a lot of good advice here already so there isn't much to add. But something specific that I try to do when I play medic is stick to three kinds of calls:
- My position
- Uber percentages (both mine and the enemy medic's uber)
- Team health
Basically I call where I am and where I'm going all the time so people know where they need to be (so I can improve my healing stats on the logs). I also call our own uber percentage a bunch and track the enemy's uber all the time (again, so I can have a higher uber count on the logs). And I call if our team health is shit, especially during fights. Because if it is, my teammates will die and I will probably die too (bad for logs). I think this is a good start for new meds. Focus on those calls and you're already a huge asset to your team.
Also something even more specific: if you're running Kritz, at 60% ask your team where the enemy med is, at 80% tell your demo to reload. At 100% laugh at your demo if he misses all his krit stickies.