Hi @Orionm45,
Welcome to the RStudio Community Forum.
Here is a minimal reproducible example (MRE) to get you started. Just cut-and-paste this code into a new R script file which you open in RStudio, and then run it. Of course, your data will be coming from your .CSV file (see help(read.csv) for examples):
a <- "
site, my_date, value
F1, 01/01/2019, 13
F1, 01/02/2019, 16
F1, 01/03/2019, 14
F1, 01/04/2019, 18
F1, 01/05/2019, 23
F1, 01/06/2019, 21
F1, 01/07/2019, 22
F1, 01/08/2019, 25
F1, 01/09/2019, 26
F1, 01/10/2019, 26
F1, 01/11/2019, 29
F1, 01/12/2019, 30
"
in.df <- read.csv(text=a, header=TRUE, stringsAsFactors=FALSE)
in.df$my_date <- as.Date(in.df$my_date, format="%d/%m/%Y")
str(in.df)
plot(value ~ my_date, type="b", data=in.df)
lines(lowess(in.df$my_date, in.df$value))
library(Kendall)
MannKendall(in.df$value)
In future, if you provide a MRE you are much more likely to get help from forums such as this.
HTH
p.s. Try to avoid using the '#' symbol in data files - it signifies a 'comment' in R and will cause you problems.