New to WordPress – questions about customizing CSS

Hello everyone,
Im new to wordpress and I have a few questions that I really can’t solve by myself, and are all about CSS coding.
1. I would really like to know what’s the easiest way to insert and modify things on my theme (CUSTOMIZR) with CSS codes. Really. Where do I insert them? Should I install a Custom CSS plugin? I’ve tried but it didn’t seem to work.

2. I would like to change the font and the color of my menu (in CUSTOMIZR theme) but I don’t know how can I do that. Could you tell me?

3. I would like to change the font and the color of the Title of my posts, but I don’t know how can I do that.
Thank you