uuh algorithm - how to make a fast code

Sorry for the strange title and code below.

How can I make a fast routine ?
Now I have something like this:

function getIt(var1%,var2%)

if var1% = a And var2% = b then var3% = c
if var1% = a And var2% = b then var3% = c
if var1% = a And var2% = b then var3% = c
if var1% = a And var2% = b then var3% = c
if var1% = a And var2% = b then var3% = c
; in the 10000 lines

return var3%
End function

How can I speed this up ?
-Must i make one big file.bb and include that ?
-Must i make a big file.dat and read that ?
-Must i use something else then If / And ?
-Maybe other ideas in the same concept

Thanks,
GC-Martijn