As a fan of TF2 I'd suggest starting with the Mod Scene. You can Google "Team Fortress 2 Modding Tools" and find a ton of information on how to make equipment, videos and more for TF2.
Starting small with a game you love and know well is the best way, in my opinion, to learn the basics.
From there you can get into more detail. I'd suggest checking out the Unity Engine
. Both have a ton of tutorials available online to help you learn the ropes.
Lastly, here's a guide I've put together to help folks navigate finding a career in games development.