Did you write any user-defined functions in your previous project? How to write and implement user defined functions in Load runner?

If you want to use user defined functions in your load runner script, first you need to define external libraries also called DLLs (dynamic link library) and store them in the Bin directory. Now create a user defined function in vugen script and pass this function name as a parameter to the DLL which is there in the Bin directory.
There is a function called lr_load_dll (), by which you can load DLLs and there by create your own functions in the script. You can normally create a DLL either using C or C++. And have to include some .h includes also. Normally one should have good C/C++ programming knowledge to write DLLs. If you are good in programming before then you can concentrate more on DLLs, else just understand the basics and leave it for programmers. As a performance engineer you need to concentrate more on analysis to find out the bottle necks.