You need to investigate
VLookup allows you to query a cell for a value (in your case the input Currency type), and look up a table to find that value and return the value in the same row in a different column in that table (in your case the exchange rate).
Indirect allows you to use a string as a reference to a worksheet location. So, if the content of cell A1 = 'X256', you can use the formula
=10*Indirect(A1) and the result will be whatever is in cell X256 multiplied by 10
Assuming the sheet your user fills in has looks a bit like this
A | B | C | D
1 ConvertAmount | Month | ExchangeCurrency | ExchangeAmount
2 25.00 | Jan | GBP |
and the rest of the sheets are named Jan, Feb, Mar, Apr etc and have the Currency type in alpha order and Currency and Rate in columns A and B respectively of each sheet.
You can use a formula like:
=VLOOKUP(C2, INDIRECT(CONCATENATE(B2, "!A1:B5")),2, FALSE)*A2
in cell D2 to get the result you require. Note that you will want to set this range to match the number of currencies you are converting from.
You might want to make sure that the user can only put in values that match your setup, perhaps using validation. For example, if the user types January instead of Jan, the sheet referencing trick won't work. Also, if the user types 'Pounds Sterling' instead of GBP, the VLookup in the other sheet won't work.