Hi experts..
pls find below my requirement scenario :
i have custom object (test_custObj) and  fileds  are ( age,street city, phone,email and contact_lookup( contact lookup field) ).  i already maintaed lookup relationship between custom object and contact .
Thrid party system sending details based on Contact ......i can able to store all the fields but not contact_lookup field . 
for example :
my third party sending details like   '23',' YG colony',' Hyd','88888888','abc@gmail.com'  and ' AMAR'.
i can able to store all the values expect 'AMAR' ( contact_lookup)
can some one help me how can i save/store  contact_lookup filed in custom object ?  
its urgent one ....thanks