Could someone take the time (ideally with a screenshot) to show which part of a pokemona stats am I looking at to find if they are valuable or not for example if I got a level 10 ratata (for example) what would I need to look at/ get. For it to be valuable

There's no one "thing" that determines a Pokemon's worth. Each pokemon needs different things that make it valuable. For instance:

 

Gengar wants a Timid nature. The only way you would know that is by looking on a website (such as Smogon) that lists what the best nature for Gengar is, or asking someone who knows more so you can learn about it..

Gengar also wants to have 31 Speed and as close to 31 Special attack. This is because Gengar is a very offensive pokemon. The defensive IV's on Gengar are mostly irrelevant.

 

However, when looking at a Pokemon like Ferrothorn, a Timid nature and 31 iv's in Speed and Special Attack actually decreases the value. Ferrothorn is a tank pokemon, so it wants a nature like Impish, Relaxed, or Sassy. Since it is defensive, we want the IV's in Defense, Special Defense and HP to be as close to 31 as possible.

 

The TL;DR of this is that the only way to understand what makes a pokemon valuable is playing the game a lot and learning, and/or looking at an external source to understand why a pokemon is good. You can always ask for help before starting to hunt for a pokemon, chat is usually helpful :)

 

Hope this helped.