trigger associateWithAccount on Contact (before insert) {
//get Company__c from Contact
//lookup Accounts by Account Name using Company__c
//get AccountID from the result of that lookup
//insert AccountID from Account into AccountID in Contact
 Map<String, String> extMap = new Map<String, String>();
 Set<String> extIdSet = new Set<String>();
 for(Contact c : Trigger.new){
     extIdSet.add(c.Company__c);
 }
 for(Account a : [select Id, Name from Account where Name IN :extIdSet]){
            extMap.put(a.Id, a.Name);
 }
 for(Contact c : Trigger.new){
     c.Account = extMap.get(c.Id);
 }
}