By referring to the image: the plot on the right is confusing, how I can detect the position of first PCA?
In the case of 2d data, the first principal component will be aligned along the direction of major continuity, and the second will be perpendicular to that, aligned along the least continuous direction. Generally just the eigen vectors at the mean x / mean y.
Sort of like so (sorry for the poor drawing)
My favorite reference on pca is A TUTORIAL ON PRINCIPAL COMPONENT ANALYSIS by Jon Shlens: pdf His example is in figure 2.
If you draw a line in any direction, you can calculate the variance of the data points' projections on that line, i.e. how much they vary in that particular direction.
If you plot the curve where the distance from the origin to the curve in any direction is equal to the variance of your data in that direction, you will get an ellipse.
The major and minor axes of this ellipse are the first and second principal components.