If my goal is to create my own model and use it in porn and I have 0 knowledge of this field (only tried blender a bit), where should I start first?
And for experienced friends, may I ask how long does it takes you to achieve that?

Welp, I'm no professional yet, but in my opinion, like any art medium, focus on your goal, but don't skip the basic knowledge.

If you have 0 3d knowledge, I'd recommend buying an online course, seeking random tutorials to practice and get used to the software.. First of all, look at blender guru videos if you haven't already, he's the best~

You'd want to master the base program first and choose whether you want to model or sculpt or some combo. Like others said, Blender is generally the go-to but it does come with its own learning curb that other software may have more or less of depending on your needs and learning style. I'd say definitely start there or similar but it's worth getting to know what more advanced techniques you'd want to prep for ahead of time if you want to get to a pro-level fast.

Some even hand-model then scan that 3D to a computer but usually this still involves cleaning up so that's one way to skip modelling but this obviously carries the expense of 3D scanning. There's also voxel-based modellers which are more like clay because they don't have the issue of triangular/quad based geometry but geometry will become an issue if you are to animate your models or use it in a game (because performance and stuff).

If Blender is to distaste (albeit its more recent improvements), definitely try other software first before giving up because some of the best modellers I know started with other software and just can't use Blender despite being pros themselves. But Blender has more tutorials for getting the basis of general modelling and sculpting (etc) techniques down.

inafox said:
Some even hand-model then scan that 3D to a computer but usually this still involves cleaning up so that's one way to skip modelling but this obviously carries the expense of 3D scanning.

I believe you can make cheap 3D scanning with just a phone nowadays.