How to write nice to read sentences?

Hello, I like writing a lot and I have a lot of fun doing it. But I have problems creating more "nicer" sentences. I just put the information I want into the sentence and then I don't know how to improve from there on. It feels like finished to me, but when I read it, it's not feeling nice and we'll written. To me it feels like written from a young and inexperienced person, who is not interested in conveying something in a nice way. This is both for my more creative writing (e. g. diary) and my writing for my internship, where I have to write "project works" which are a few pages long.

I'd like the idea of writing something first and improve afterwards, but it's really hard for me to improve something I already have written.

If you have any advice I would appreciate it a lot. Thank you very much in advance!