Check out this website which shows you how to change the plot title, the axis labels, and the legend title:
http://www.sthda.com/english/wiki/ggplot2-title-main-axis-and-legend-titles
This second page shows you how to add annotation at specific positions and labels for individual points:
http://www.sthda.com/english/wiki/ggplot2-texts-add-text-annotations-to-a-graph-in-r-software
For how to change the background colour, see here:
https://www.datanovia.com/en/blog/ggplot-theme-background-color-and-grids/
p.s. Some simple googling would have got you these results!
HTH