Basic Pantheon Guide

Basic Rune Choices

Red - AD (early game advantage for spear harassment)
Blue - MR (magic res to ensure survivability in mid)
Yellow - ARMOR (mid/late game survivability)
Quint - MOVE (early game advantage to get in range to spear and stun)

The item selection will vary depending on who you are playing against and how well you are doing.

Hex/Merc - this should give you an early game advantage vs the AP champ you will face mid

Early - Brutalizer can be substituted for a quick bloodthirster
Mid/Late - Guardian Angel might be useful for survivability
Mid/Late - Infinities edge will greatly increase Panths damage but sacrifices survivability
Mid/Late - Triforce is a good alternative to ghostblade

General Tips
E (passive) will help with last hitting.
Your ulti can be used to force a fight when ganking a lane by closing off escape routes.
E does a stupid amount of damage late game in team fights, don't underestimate it.

Early Game

Panth has a huge advantage vs an ap mid, you do way more early game damage and are terrifying.
You need to Q spam as much as possible (Try not to fight in the middle of minions when you w). If your opponent is playing passive and wont come out, freeze the lane and don't let them farm.

Early/Mid

After level 6 you need to closely watch the map and see which lanes are pushed, ping and leap and hopefully your team responds to the gank.

Mid

If you are ahead of mid on farm you should be ganking as much as possible even when your ulti is down (hopefully your team alerts you to where the wards are).

Mid/Late

You can use your ulti to initiate fights, which might force the enemy team to disperse in different directions (giving your team the advantage in a team fight). Make sure to focus the carries as you can essentially 3 shot them.