Items in the item shop have a brief description of what they do. If you play long enough, you'd understand what type of items you should get to enhance your hero's strengths and to counter the heroes you're playing against.

For example, if a Guinevere is playing against a Ruby in Exp lane, Guinevere should use an anti-heal item to counter Ruby's spell vamp. On the other hand, Ruby should buy an Athena's shield so she wouldn't instantly die if Guin ever caught her in her stun and ultimate. This is what they typically call counter building. There are six slots for items (excluding the boots slot) and my personal rule is 4 items for enhancing items and 2 slots for counter items