You need to have a table someplace in a worksheet that has all the info you will later want to reference.
You then reference that table in your lookup formula.
If you want "Jack", "42", and "981571" to appear when you type "Smith", then "Jack", "42", and "981571" all has to be in a table someplace to refer to.
If the info in Excel's help files isn't clear, a google search will list a multitude of primers, examples, and explanations on vlookup, hlookup, index, match, etc. (all methods of locating data in a table).