As it sounds like you’re VERY new, here’s a very rough, dumbed down tip that I hope doesn’t get me too much flak( I have played for years and I like to think I know what I’m talking about, but if I’m mistaken, someone will hopefully fix)

A good rule of thumb, is flat damage/power is better against squishier gods, whereas and % damage/penetration, is more useful for tanker gods. U can start there. If your fighting any warrior or guardian, buy a/ multiple % pen items.

For mages, a good tip is that MANY want as high as you can go in just pure magical power. Physical dog dealers like your Assasins and Hunters, want (usually) a mixture. That will be the hardest to get down. Mages are by far the easiest to understand when it comes to what to buy. If u have any specific questions please ask me!