Flight models are probably the hardest aspects of modding - which is why there are very few who actually understand the variables involved and create them. These select few probably have a background in physics/aerospace engineering (Like TK) and the advanced math involved.
Yes you can change the variables involved and go through a trial and error process - but this is very difficult and painfully time consuming especially when you see the amount of variables and dont understand how they link together - and the results can send you in all different directions!!.
Avionics, engines, and weapons parameters - again involves changes in ini files and creation of image files like tga/jpegs (Photoshop/GIMP). Like the flight model due to the amount of variables involved it is not always easy to get what you want even after a lot of trial and error.
Computer programming - this is only relevant if you are creating an external program like the weapons editor or the old mission editor - luckily you do not need any programming skills to alter the actual games - i.e. change .ini files or create 3D models or 2D skins.
Decals and Skinning - start by altering existing skin files in Photoshop or the free GIMP program and learn from there.