=VLOOKUP(What, Where, My column, TRUE or FALSE)
What = what you are trying to lookup. This can be a value (e.g. "Apples") or a cell reference (e.g. C197).
Where = the range of cells that you are looking in (e.g. A2:V302). If you are looking for values on another sheet you need to use the syntax 'My other sheet'!A2:V302.
My Column = the column number of your range that you want to return a value from. So if your What is in Column C and you want to return the corresponding value from Column F, you specify Where as e.g. C1:F30 and My Column should be set to 4 (C is 1, D is 2, E is 3, F is 4).
TRUE or FALSE specifies whether an exact or approximate match is required. FALSE looks for an exact match - if one is not found #NA will be returned. If you specify TRUE, or omit this parameter, VLOOKUP will try to find the nearest match. Your data must be sorted for this to work. For example if you have a table of temperatures and refractive indices, this must be sorted with temperature increasing or decreasing depending on whether you want to return the next lowest or highest value of RI respectively.