To start
Go for regrowth pendant and health potion to start. Always be cautious seeing as you don't have any escape abilities and you don't do mass damage at the beginning. But you are still a threat with your spear and stun abilities. Focus on a single champion first, preferably a healer if there is one present. If not, then the most squishiest.

Next
Purchase the BF Sword. Sure it will take a lot to build to but once you achieve this item and hopefully only die once at the most, you will be a much bigger threat now with you abilities.

The boots
Of course go for these boots unless you are facing an all AD team, which will most likely never happen.

The life steal
Now you can achieve the bloodthirster, a very powerful weapon for pantheon. Try to feed off of jungling first before engaging in team battles.

The tank item
Now comes you warmog's armor, the tankiest of the tank items. It will now be very hard to face you one vs one. They will most likely need another champion to bring you down, which is of course what you want. Seeing as you are still deadly even with just one bloodthirster and yet a tank as well.

Final build up
Now it all depends how well you do with your health at this point. If you start to see it drop to rapidly in fights, then I suggest you go for the sunfire cape or banshee's veil. It all depends on which one is hurting more. If you don't struggle, then shoot for your second bloodthirster. In the end with this final build, you are deadly, depending on whether they counter your build, and also a huge tank, regardless of their build.

Final thoughts
I've used this build many a times and kicked *** with few deaths. I become the tank or the second tank, with still some kick.