20 level Paladin spells
 

my DM, gave me a shield that gives me a 20th level paladin spells, is there a way I can add this item to my character which is a fifth level wizard

I am sorry what exactly does that mean?
1) You gain 20 levels of paladin for purpose of casting spells?
2) You gain all the spells of a level 20 paladin including the casting slots?
3) You gain access to paladin spells that you can add to your spellbook?

Its just not that clear and it really makes a difference in how to answer your question...

option two

I gain the spells and casting slots for a level 20 Paladin
+ 5 Ac against fire

So I assume these new spells will use your Int to calculate its DC. So my best advice is to make an archetype that uses the "Spellbook Bonus Spells" feature to add those "paladin" spells to your spellbook. Then use "Spells Cast Per Level" to adjust for the extra spell slots you get.

That would be the easiest method without actually getting into writing scripts. If your new to the editor then see FAQ#2 for learning the editor and for the four intro videos.

I think I know the shield you're talking about. I'd run with ShadowChemosh's suggestion, as anything else is likely going to be a pain.

Who would make a shield like that?

Paizo - APG pg 317 - The Shield of the Sun