how to get the one entity value and set it to a lookup ?

Hi All,

I have SLAPenalty field on an Agreement entity.

I have workorder entity with Priority{ [lookup]-(name field)} and agreement field {[lookup]-(agrrement name)}

Now i wrote the code to get the penalty value through agreement 

based on the penalty value i have to set the priority {loookup value}

{Requirement}--i have to copy the penalty value to priority lookup if the user given penalty value is not already existed in the priority....If the usergiven penalty value is already existed no need to set that value to the priority lookup again....

I want to know how to write the code to search for the penalty value in the priority lookup

Please help me with this... I have to do this with c# Plugin not JS

Thanks,

Maanasa Gutha