You can't simulate fabric directly inside AE very easily. You can with FreeForm Pro or Zaxwerks Flag or Element 3D or others. Blender, a free open source 3D app has a very good fabric simulation that's easy to use. You could render the falling flag in Blender and just import it. Unfortunately C4D Lite, that comes with AE does not support fabric simulations so you'll have to find another source.
If you're really up for a challenge and you are very good at figuring out displacement and understand displacement maps, blend modes, distortion effects and all of that you can 'fake' an unfurling flag with a bunch of layers and a bunch of experimenting. If you're new and don't have access to a real 3D app I'd just head over to blender.org and give that a try. They have lots of good fabric simulation info in the site. Modeling with the Cloth Simulation
Thank you so much for your response.
Although, I didnt understand what you said about Other source ?
It's easier for me to work on 3DS Max. Never workedon blender. Do you think it's better ?
I tried to do it via AE by using Fractal noise, by following a tuto. In the tuto the guy used Card Dance after Fractal effect, and thats where I just stopped... it wont work for me...
is it doable to animate on 3D max and import it to AE ?
thank you !!
If you have 3DS Max use that. I don't know anything about you and only suggested Blender because it's very powerful and free. It's a little wonky and takes time to learn but if you have no budget and some time it's probably the most capable 3D app out there for free.
Just export an image sequence with an alpha channel (straight alpha) and import it as an image sequence. Just make sure you check the frame rate and make sure that it matches your comp and the other footage in the project.