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I wrote a few blog posts earlier this year on how to do some of this. Creating thresholds, canny edge detection, finding contours, etc.

• I'd start here: https://www.ccoderun.ca/programming/2018-06-05_OpenCV_2/
• Then look here: https://www.ccoderun.ca/programming/2018-06-07_OpenCV_3/

In case my site ever goes down, here is a bit of example code taken from those pages. But I strongly suggest you read through the post if possible prior to blindly trying to use the code.

#include <opencv2/opencv.hpp>

int main(void)
{
    cv::Mat original_image = cv::imread("capture.jpg", cv::IMREAD_COLOR);
    cv::namedWindow("Colour Image", cv::WINDOW_AUTOSIZE);
    cv::imshow("Colour Image", original_image);

    for (double canny_threshold : { 40.0, 90.0, 140.0 } )
    {
        cv::Mat canny_output;
        cv::Canny(original_image, canny_output, canny_threshold, 3.0 * canny_threshold, 3, true);

        const std::string name = "Canny Output Threshold " + std::to_string((size_t)canny_threshold);
        cv::namedWindow(name, cv::WINDOW_AUTOSIZE);
        cv::imshow(name, canny_output);
    }

    // ---------------

    cv::Mat canny_output;
    const double canny_threshold = 100.0;
    cv::Canny(original_image, canny_output, canny_threshold, 3.0 * canny_threshold, 3, true);

    typedef std::vector<cv::Point> Contour;     // a single contour is a vector of many points
    typedef std::vector<Contour> VContours;  // many of these are combined to create a vector of contour points

    VContours contours;
    std::vector<cv::Vec4i> hierarchy;
    cv::findContours(canny_output, contours, hierarchy, cv::RETR_EXTERNAL, cv::CHAIN_APPROX_SIMPLE);

    for (auto & c : contours)
    {
        std::cout << "contour area: " << cv::contourArea(c) << std::endl;
    }

    const cv::Scalar green(0, 255, 0);
    cv::Mat output = original_image.clone();
    for (auto & c : contours)
    {
        cv::polylines(output, c, true, green, 1, cv::LINE_AA);
    }
    cv::namedWindow("Contours Drawn Onto Image", cv::WINDOW_AUTOSIZE);
    cv::imshow("Contours Drawn Over Image", output);

    // ---------------

    cv::Mat blurred_image;
    cv::GaussianBlur(original_image, blurred_image, cv::Size(3, 3), 0, 0, cv::BORDER_DEFAULT);

    const size_t erosion_and_dilation_iterations = 3;

    cv::Mat eroded;
    cv::erode(blurred_image, eroded, cv::Mat(), cv::Point(-1, -1), erosion_and_dilation_iterations);

    cv::Mat dilated;
    cv::dilate(eroded, dilated, cv::Mat(), cv::Point(-1, -1), erosion_and_dilation_iterations);

    cv::Canny(dilated, canny_output, canny_threshold, 3.0 * canny_threshold, 3, true);
    cv::findContours(canny_output, contours, hierarchy, cv::RETR_EXTERNAL, cv::CHAIN_APPROX_SIMPLE);

    cv::Mat better_output = original_image.clone();
    for (auto & c : contours)
    {
        cv::polylines(better_output, c, true, green, 1, cv::LINE_AA);
    }
    cv::namedWindow("Another Attempt At Contours", cv::WINDOW_AUTOSIZE);
    cv::imshow("Another Attempt At Contours", better_output);

    cv::waitKey(0);

    return 0;
}