There is this good guide from
YameteOuji https://hub.virtamate.com/resources/t-shirt-guide.953/ .
I started with modeling clothes 3 weeks ago and work only with the free 3D software blender (free). You need DAZ 3D (free) too to get your 3D model to VAM. Additional some skills in 2D grafiks are necessary: Photoshop or GIMP (free).
To create my first piece of clothing ready to publish in VAm I spent about 100 h in learning, trial and error, watching tutorial videos for blender etc. And I'm still learning more. Its not that easy if you start from zero. But its alot of fun.