Practice practice practice.
That is the best tip I could give to you. And aside from that as others already mentioned a sculptingmedium (I prefer Magic sculpt) Tools are actually not something you have to invest immense amounts of money in. I made most of my tools self. Think of a needle with a wooden grip, Toothpicks all differently shaped heads ranging from sharp to blunt and dipped in CA glue to make the Magic Sculp not stick to it anymore. And the great partytrick of Magic sculp is that you can smooth it with water. It is almost like painting the folds on.
Easiest to start is to convert a figure that is close to the pose you require. I did this a while ago when I needed a racingcar driver from the early days of racing. Took a German Panzercar driver and converted him. This was the result. Left was the source figure, Right was halfway in the conversion.
And here is the figure primed next to his ride
Then of course completely scratchbuilding a figure from scratch is a whole different branch of sport. Usually this starts with a wire armature. Which is bend to shape. I usually cut corners a bit by using hands boots and heads from existing brands such as verlinden, hornet etc. And then you start building up.
Here is an example of an Armature
Then starting to flesh it out
Adding clothes
Untill you end up with something like this
And then you get this scene where he is with his buddy who is also specially made for this project
Even with the leftover Magic Sculp I keep experimenting
This is a figure I sculpted before the Kettenkradrepairmen
All I can say if you want to go down that route (and I am by no means an expert on the subject) is buy the right medium, Study photographs of clothing. Prefferably the clothing you want to sculpt. Follow some basic anatomy rules (or kinda copy the dimensions of a Dragon figure, that is what I usually do) and don't be dissapointed if it doesn't turn out the way you want it the first time around. Or the second time or even the third time. I have created stuff that would even be rejected for zombiemovies. But if you can pop out a decent figure there is no diorama idea that can't be done anymore.