Do I need an expert to fix the issue or can I fix it myself?
Well, if you understand what the PageSpeed Tool from Google is telling you then you can fix it by yourself, I guess. And if not then you need an Expert. 
Google has links to all points and explain them in detail. But it’s very technical.
The biggest problem is the Javascript Code. Either the Code of your theme is very very very inefficient (because theme developer didn’t know what he did). And if it is a standard theme then this might happened because of custom modifications. Or if no one touched the theme code and it’s a Shopify standard theme then you have too many Apps installed. Every App costs performance/speed if it changes something visible for customers (easily speaking).
Try removing all apps you really don’t need for boosting your sales.