Short distances, no "air"
Usually its ok to use the palette from 3 colors. Plus 2-3 colors could be the lighter shades of the colors you already have.
Make sure the colors you are choosing match each other. It is easy to do through the many websites like colorhunt.co
It is better if we use one style - for example graphic cartoons or pictures. When you combine both - that is already not good sign.
Also be careful about icons - they should be from one set to much each other.
Also you can only have max two fonts on the website - 1 for the headlines, and 1 for the main text.
All the buttons on the website should be the same. Sometimes you want to bring attention to one particular button.
Ok, it could be for example bigger, or another color (from your palette) but it should have the same style as others (text color and font, stroke or not, shadow or not).
The amator web design you can always see in the attempts to fit everything in one screen using too many elements and leaving very small space between them and between the screens (I used to do it as well).
Don't be scared to leave bigger gaps, leave space and the air - it is always better. Also make sure the distances between similar elements are the same (for example: between heading 1 and heading 2, between each screen, ect).
Too many different styles and fonts
There are many ways to show your business on the web. But before you make a structure check the websites of your competitors, the best ones on the market.
If they grow - its just works, so take it and use for yourself.
when you do the website yourself
*Check your website and make it looking more professional. Or ask professional to do it.