how do i use vlookup function?

The VLOOKUP function has four arguments:

VLookup( value, table_array, index_number, not_exact_match )

lookup_value:
What value do you want to look up? In this example, the product code is in cell A7, and you want to find its product name.

table_array:
Where is the lookup table? If you use an absolute reference ($A$2:$C$5), instead of a relative reference (A2:C5), it will be easier to copy to formula to other cells. Or, name the lookup table, and refer to it by name.

col_index_num:
Which column has the value you want returned? In this example, the product names are in the second column of the lookup table.

[range_lookup]:
Do you want an exact match? Is an approximate match okay?
If you use TRUE as the last argument, or omit the last argument, an approximate match can be returned. This example has FALSE as the last argument, so if the product code is not found, the result will be #N/A. (Note: Excel is rather forgiving, and will accept 0 instead of FALSE, and 1 instead of TRUE.)
also try EXCELs HELP