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Please, take a look at this question. So you could do something like this:

SPListItem lookedUpItem =  GetItemSomeHow();
newItem[lookupField] = new SPFieldLookupValue( lookedUpItem.ID, lookedUpItem.Title);
newItem.Update();

Hope it helps. If so, please, don't forget to press upvote and mark as answer. :)

Regards, Michael (http://sp2013-blog.com)