Really strong English. Some good corrections already. I'll offer some suggestions, not corrections per se, but rather how I would say this as a native speaker. Hope it's helpful.

Introducing myself

Hi! I'm Erica. I'm Italian, twenty-four years old, and starting my fifth year of medical school. I understand English very well, both written and spoken English, but I struggle a bit with speaking and (as you can probably see) with writing. I usually use an online Italian-English translator for some words or idioms here and there, but this time I going to try writing without it. I'd love to get any feedback on some of the main things you see that could help me improve my writing.

Be gentle!.. :)

Thanks so much, 
Erica

Introducing myself

Hi! I'm Erica and I'm Italian. I'm 24 and I study medicine, I'm starting my fifth year. I understand English very well, both written and spoken, but I have some trouble in speaking it and (most of all, as you can see) in writing it. Usually, I use an online translator which translates Italian into English for some words or idioms, but this time I want to write without it, to know better what I need to improve.

Be kind to me and my mistakes. :)

Bye,
Erica

have trouble doing something = have difficulty doing something. There is no preposition after trouble

To translate something INTO something else.

I study medicine is correct, you don't need to use the present continuous.

Good luck on your career!

Very good writing! I made some suggestions for you to consider...

Hi! I'm Erica and I'm Italian. I'm 24 and I'm studing medicine. I'm starting my fifth year. I understand English very well, both written and spoken, but I have some trouble in speaking it and (most of all, as you can see) in writing it. Usually, I use an online Italian-English translator for some words or idioms, but this time I wanted to write without it, to learn more about know best what I need to improve. 

Be kind to me and my mistakes. :)

Bye,
Erica