Finding HTML Errors In Posts

I am new to WordPress, and CSS code so would appreciate advice in layman’s terms! My sidebar content is now appearing at the bottom of my webpage, instead of on the right hand side where it should be. I understand from reading online troubleshooting posts that I probably have an HTML error, which is very possible because I recently pasted my google analytics code into CSS. However, as someone who has no clue how to fix this, or I don’t know how to find or correct these errors.

Any guidance is very much appreciated.