First minis feedback/help

Hi, I got the initiation set for my birthday (5 space marines and 11 tyranids).

That's the first time I painted something (other than in school when I was 5 maybe), so I'm looking for advice/feeback.

I linked images of my 3 space marines, the first one was the very first mini (not counting the tyranids and with only initiation set paints), the second one is after I bought some paint and tried using agrax earthshade for shading and the third and final one is the sergeant where I tried shading + highlighting.

For the second/third I tried going "outside" the initiation set steps for some components like pouches and metal just to try "improvising" by myself for future minis.

I watched tutorials but I think I don't really grasp how highlighting is supposed to be done or where. I'm not satisfied with my first try on the sergeant.

For shading I understand it's supposed to be in the recesses and metal but I think I put too much paint ? Or I'm not accurate enough.

Do you have any advice for my next minis or feedback for the painted ones ?

Feel free to be harsh I don't mind !

Thank you !

(Don't know if the pictures are good enough)

Link for the pictures because the post was too long : https://ibb.co/album/TYgGL0