A formula tool will get you there. This formula should work for that example to create [newfield]:
IIF(StartsWith([data],'124'),[data],[newfield])
If you want multiple conditions then use the same principle but If Then Elseif Then Elseif Then Else ENDIF rather than IIF()
Oh, and if you want to split 124000023 into 124, 000023, then use left([data],3) and Right([data],Length([data])-3)