.tga models? .tga is typically used for textures, not model meshes...

either way, most people use Blender, or 3DS Max. there are some older model editors that support Source modelling, but I wouldn't recommend using those.

1. get Blender or 3DS Max and learn the basics of the editor you pick
2. get accompanying plugins and learn the basics of compiling models: https://developer.valvesoftware.com/wiki/Compiling_a_model - as for model sources, you'll have to find those yourself
3. if you want to make car models: https://developer.valvesoftware.com/wiki/Vehicles_(modeling) + I'd recommend repurposing the Half-Life 2 Jeep's source code from the Source SDK

it took me over a year to go from barely knowing how to compile a mildly broken model to compiling a functional car model, so you'll have to be a bit patient.
note that it'll likely be quite confusing at first.

if you want to port from Forza, you should use Forza Studio, which works with any model editor that can load .obj models. as far as I know it works with all the games up to Forza 6 Apex and Horizon 3, because those games use a new model system which hasn't been fully figured out yet.

if he thinks that tga is model filetype then he won't succeed in porting anything imho
but yeah, thanks for your answer, it's comprehensive

thanks for that,i just never used tga before only .cad and .obj files

there's more btw: Driver San Francisco, Assetto Corsa, Need For Speed.