Hi@akhtar,
You can create a folder at the time of creating an S3 bucket. You will find one resource aws_s3_bucket_object that will help us to create one folder inside the S3 bucket. I have attached one example below for your reference.
resource "aws_s3_bucket_object" "folder1" {
    bucket = "${aws_s3_bucket.b.id}"
    acl    = "private"
    key    = "Folder1/"
    source = "/dev/null"
}