I doubt any one person will be able to tell you "how to develop cheats and hacks on online games"
I mean there's people out there that could likely explain in great detail, but your question is not very specific, and it would take years to explain all there is to know.
I've only been doing this for a few months now, although i have been programming for a few years, i'm no pro in this particular area but my advice is start small. Learn c++ and then start on the basics of memory manipulation. Learn what offsets are, why they are relevant, and how to find them. Once you get that down, you can write some code that manipulates that memory. Then move onto something like signature scanning.
Writing cheats for multiplayer means finding ways to bypass or trick Anti-Cheat, and that's one of the last things you should be worried about if you're just getting started.
Trust me, you don't want to jump in to the deep end if you have little or no experience, as you will find yourself getting very frustrated.