I assume you want to start with malware development? C C++ AutoIT VBS Batch are a good start, I specifically mention those for Windows OS, but I'm sure there's plenty more you can do. I am somewhat a noob and just got done developing a pretty fancy piece of malware by studying open source malware. I can only compromise PCs with Windows Defender, but that is soon to change. I came up with a subtle method to run my malware without making too much of a ruckus and getting too much AVs attention. Its a slow process BUT IT WORKS. I guess first, you have to think of what you want to achieve with your new acquired knowledge, then sail off on a project to polish your skills. Hackforums does a good job at leaking source code to people's malware LOL. I've come across some weird looking stuff searching lately. HAHAHA Hope this helped.