thank you guy, the pictures are beautiful, what camera did you use ?
so any modern camera can avoid lense distortion?
There are different types of distortions produced by different lenses. There are radial distortions ( mostly barrel and pincushion) that are minimal or negligible with good lenses and can be easily fixed these days in post processing or even by the camera engine itself. That is not what you need to be concerned about at this stage.
There are also perspective distortions caused by the focal length of the lense in relation to a sensor size used by the camera. That is what you should be concerned about when shooting people. Wide lenses distort faces simply because they distort perspective, this is not a fault, this is how they are made on purpose to have a wide field of view, and you can not correct it in post processing. You just need to use longer focal range lenses.
With APS-C sensor camera you can take great portraits with 50+ mm focal length, although most photographers will tell you that something like 85 mm would be more desirable. It all depends on what kind of portrait you are after. With a larger sensor you will need a longer focal length. With smaller sensors shorter focal lengths will do. For a group portrait you will need a somehow wider lense, for a close-up you will need a longer focal length lense, like 85 or 120 mm on APS-C sensor camera.
Talking about sensors, you should avoid cameras with a small sensor, not because of distortion, but because of poorer image quality. You should buy a camera with a sensor that is at least 4/3 (most mirrorless cameras), or even better, APS-C size (some mirrorless and all beginners and intermediates DSLRs) . You can go all the way and buy a full frame camera, but it will be big, heavy and will cost you a lot. But these are nice cameras. You can google "sensor sizes" to get familiar with this, it is important to know
I personally would recommend a DSLR camera with APS-C size sensor. It is relatively not expensive, gives you high quality images and have great variety of lenses. Having shot different cameras, I personally would be torn between Nikon and Pentax, both excellent modern cameras. Canon would be my distant third choice, but it is also a very good camera.
All these cameras will give you amazing image quality, you will not be disappointed, guaranteed. I shot those images with FUJI X-T1 which I prefer to all those cameras, but it costs more.
Good luck. Get a camera, start shooting people, post your images here for comments and critique, and there are some top portraiture pros on this forum that will guide you in the right direction
AS for particular technique, it is impossible to explain in a forum post, buy something like this
Portrait Photography From Snapshots to Great Shots Amazon.co.uk Erik Valind 9780321951618 Books
it will entertain you for the next year or two.
This one is also highly regarded:
Direction and Quality of Light Amazon.co.uk Neil van Niekerk 9781608955701 Books