In order to test the trigger you need to insert records that will cause the trigger to fire.  You need to create a class to do this.  Because this is a test (note the static testMethod) no data is actually committed to the database.  Below you will see that I create an account and set the Market to GBR, then I insert the account.  Once inserted, I test to see if the trigger set the value of Iso code appropriately.  I then continue to do that to ensure that all lines get tested.

@isTest
private class testBillingCurrencyTrigger {

    static testMethod void myUnitTest() {
		//Create an account to insert with Market = GBR
		Account acct1 = new Account(Name='Test Account', Market__c = 'GBR');
		insert acct1;
		//Test to see that the trigger fired and changed the CurrencyIsocode go GBP
		System.assertEquals('GBP',acct1.CurrencyIsoCode);
		//Set Market to ESP and insert again. keep doing this to ensure we test all lines
		Account acct2 = new Account(Name='Test Account', Market__c = 'ESP');
		insert acct2;
		System.assertEquals('EUR',acct2.CurrencyIsoCode);
		Account acct3 = new Account(Name='Test Account', Market__c = 'CAN');
		insert acct3;
		System.assertEquals('CAD',acct3.CurrencyIsoCode);
		Account acct4 = new Account(Name='Test Account', Market__c = 'CHE');
		insert acct4;
		System.assertEquals('CHF',acct4.CurrencyIsoCode);
		Account acct5 = new Account(Name='Test Account', Market__c = 'CZE');
		insert acct5;
		System.assertEquals('CZK',acct5.CurrencyIsoCode);
		Account acct6 = new Account(Name='Test Account', Market__c = 'DNK');
		insert acct6;
		System.assertEquals('DKK',acct6.CurrencyIsoCode);
		Account acct7 = new Account(Name='Test Account', Market__c = 'NOR');
		insert acct7;
		System.assertEquals('NOK',acct7.CurrencyIsoCode);
		Account acct8 = new Account(Name='Test Account', Market__c = 'SWE');
		insert acct8;
		System.assertEquals('SEK',acct8.CurrencyIsoCode);
		Account acct9 = new Account(Name='Test Account', Market__c = 'USA');
		insert acct9;
		System.assertEquals('USD',acct9.CurrencyIsoCode);
		Account acct10 = new Account(Name='Test Account', Market__c = 'AAA');
		insert acct10;
		System.assertEquals('GBP',acct10.CurrencyIsoCode);
	}
}

Good luck!