This is my first serious website. It is obviously not professional, but please tell me how to improve. Thanks for all constructive criticism/tips on what to learn next :)
Your header is way too large. The user has to scroll down to see any idea of what the website is for. I like the picture of the boy kicking the ball, but that should be near the top of the page, and you should have a navigation bar above that picture. Make the logo a lot smaller and put it in the top left corner. It's not good to display a hamburger menu on the desktop.
I would make the logo and title smaller since it takes up so much space of the viewpoint on the initial load. Look at other well-established websites and follow their design guidelines. Check the height of their navigation bars. They poured millions into research and design, you can easily benefit from their hard work.
I would place a hamburger menu icon next to the menu text or replace the menu text in general with an icon. Everyone knows what a hamburger icon represents by now. Make the icon and text smaller.
Change the font text size in the footer and make the map bigger. Put the map above the footer text.
Since you're new to the game, Google chrome has an inspect function that allows you to check the CSS of a page. Do this for the bigger websites to check their design guidelines.