For items u get ad and critical and attack speed
Reason why u don't get health cuz shaco is not a tank

Shaco is an assassin champ
So u either die first or kill first when u in the fight

For the 2nd item the BOTRK is really useful making ur opponent no escape
Ur passive when u first hit them u slow them down. However, the slow effect is not long enough
That's why the E (throw knife) is ur 2nd chance to slow them down but for some ppl they don't use the E until they know they can kill them with the throw knife
For most ppl they throw knife ONLY when they can't catch up their opponent
That's why BOTRK is the 2nd chance for u to slow them down during the fight

For the defense which is the 5th item u either get thornmail, GA, or veil. It's depend on ur opponents.
Example like if ur opponent's team have a lot of AP and their carrier is AP or they have a lot of cc u get banshee veil
If u don't know wat u want for the defense just grab a GA

Most ppl use flash for shaco
But for me i think ghost is better

The reason u use ghost so u can run faster like in and out of battle faster
Cuz ghost work with the Q while u in stealth
Example like 3 ppl chasing u, even when u use ur Q u won't escape successful cuz 1 of the 3 ppl will eventually catch up to u. During this situation u use ur ghost. When u in stealth they lost track of u. When u use ur ghost while u in stealth eventually there will be no way for them to chase u

You can also do the same for engage