Assistance w/ ROM cheats/hacks/modifying?

[Its_Stephenn]

Hi there

Please excuse my naivety if this question is incredibly easy to execute, I just haven’t been able to find a guide to do what I’d like or anything similar and I’m very new to the world of emulation.

My first question, when I’ve looked up ROM hacking or modifying it seems PkHex is a commonly occurring software, what actually is it and how would I get it - providing it is useful for my next question -

I have just downloaded the Desmume emulator to run SoulSilver. I would like to be able to use some little cheats throughout the game for fun, such as solely modifying my next encounter to be a specific Pokémon? Or making it so that the next Pokémon I encounter is shiny? Or tweaking my money count etc? Enabling myself to have more master balls and so on? 
 

Again, I’m sorry if these questions are blatantly obvious things to do but I’d really appreciate some help, I have no clue where fo start!

Thanks so much

Stephen