I am a junior developer and have been given a wordpress job to do. I am perfeclty capable on standard CSS html however wordpress is driving me mad as I cant seem to control what I want to - fo rexample my blog post wont center with width:100% , margin 0 auto. Is there a crash course which will help me understand how this platform works?
It's quite straight forward. Either make a child theme, which is useful if you're just modifying an existing theme or just make a new theme which is almost as easy. As mentioned above, it's the same as any old CSS markup.
I've always found the WordPress Codex very good:
https://codex.wordpress.org/Child_Themes.
If you're a junior developer then I'd expect you know everything you need to know to make a WP (child/full) theme, so just get stuck in with the Codex!
I'd recommend you start by making a child theme off the main theme, or off a theme that looks similar-ish to what you want. Then when you get the hang of that, it'll be easy to make your own theme from scratch if you want.