Still a low level.
League does a pretty poor job when it comes to beginners. Some mobas do even worse.
You could check out the wiki page by googling. Or just read some third party information.
Generally, it just comes from a lot of experience.
HP and MR is good against AP damage/ Bad against true damage and percent health damage
Armor is good against against basic physical damage
Grievous wounds and armor penetration help against enemies with a lot of armor and regeneration.
Play some aram.
Look at the damage you took after you died.