Lookup - how to do this

There are two sheets, sheet 1 and sheet 2
Sheet 2 already has some entries.
Now i want to enter some alpha-numeric values in sheet 1.
What i want now is, if the value being entered is found in sheet 2, it

should not be allowed to enter.
ie. while entering, it should automatically search in the given range of

sheet 2 and if the value is already there, it should give warning or

ZERO as answer.

how to do this?

Thanks.